Choosing a Roman name
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The system of Roman names was unique and distinctive in the ancient world. When a foreigner became a Roman citizen he took a new Roman name as a mark of citizenship. In Nova Roma, too, we ask that you choose a Roman name when you become a citizen.
The name you choose is the name you will be known by in Nova Roma, so choose carefully and seriously. These pages contain information to help you. If, after reading this, you have any more questions, please contact the Censores.
Contents
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Tria Nomina
Most Romans have three names (tria nomina):
- a praenomen (plural praenomina),
- a nomen (plural nomina), and
- a cognomen (plural cognomina).
A few have no cognomen; a few others have more than one cognomen.
Praenomen
Your praenomen is a personal name which distinguishes you from other members of your family. You will not normally be called by your praenomen on its own: normally only close relatives or very close friends call each other by their praenomen alone. (See below under "Usage" for more information on how names are used in conversation.)
There are only a small number of praenomina in ordinary use. These are used by the vast majority of Novi Romani, as they were by the vast majority of ancient Romans.
There are also some rarer praenomina, most of them very old ones. If you have some good reason to want a rare praenomen, you may be allowed to have one, but such requests are very rarely granted and must be personally authorized by the Censor. We strongly recommend that you choose one of the standard ones.
Abbreviations
Note that each of the common praenomina, and some of the rare ones, has a standard abbreviation. Each abbreviation is unique to that praenomen: you cannot abbreviate a praenomen simply by using its first letter. For example, T. always means Titus, never Tiberius; Ti. always means Tiberius, never Titus.
Most of the time praenomina are abbreviated rather than written out in full, so you will normally see M. Tullius Cicero rather than Marcus Tullius Cicero.
Inheritance
Many families use only a handful of praenomina. The first child of a marriage is almost always given the name praenomen as his father; the second child will be given a different praenomen, perhaps the same one as his uncle or grandfather, for example.
Thus the elder son of P. Cornelius Scipio was named P. Cornelius Scipio (Africanus); his younger son was named L. Cornelius Scipio (Asiagenus) after his grandfather.
List of Standard Praenomina
These are the standard praenomina, from most common to least common.
List of Standard Praenomina
These are the standard praenomina,
from most common to least common.
Abbreviation | Male Form |
Female Form |
C. | Gaius | Gaia |
L. | Lucius | Lucia |
M. | Marcus | Marca |
P. | Publius | Publia |
Q. | Quintus | Quinta |
T. | Titus | Tita |
Ti. | Tiberius | Tiberia |
Sex. | Sextus | Sexta |
A. | Aulus | Aula |
D. | Decimus | Decima |
Cn. | Gnaeus | Gnaea |
Sp. | Spurius | Spuria |
M'. | Manius | Mania |
Ser. | Servius | Servia |
Ap. | Appius | Appia |
N. | Numerius | Numeria |
Nomen
Your nomen indicates which gens you belong to. A gens is a loose collection of families sharing the same nomen. In ancient times a new citizen would almost always join an existing gens, and similarly in Nova Roma we ask you to choose a nomen from a closed list.
Inheritance
A Roman always takes the nomen of his father.
List of Nomina
The nomina are listed in their male forms. To make the female form, just replace the ending "-us" with "-a".
Some ancient Roman nomina which are not listed here may also be acceptable. If you want to use a nomen which is not on this list, the Censores will consider your request.
To see the members of a gens, or other details of a gens, see the Album Gentium.
Cognomen
Your cognomen is a family name which would be shared by your blood relatives. Cognomina often refer to a person's appearance or other characteristics, but they do not have to. It is quite common to have a cognomen referring to a place of birth, a job, or some other thing which distinguished the person (usually an ancestor) who first bore that cognomen.
Almost everyone has a cognomen, and it is difficult to distinguish different families within a gens unless cognomina are used. However, strictly speaking, cognomina are not compulsory. If you don't choose one when you first apply for citizenship, you will still be able to add one later.
To help you choose a cognomen, there is a list of ancient Roman cognomina below. But this is not a complete list - the Romans often created new cognomina, and if you want to have one which is not on the list we will be happy to discuss this with you.
The Characteristics of Cognomina
Ancient republican cognomina had certain general characteristics. A person was not given his cognomen by his parents and did not choose it for himself: he either inherited it from his parents or was given it by general consensus within the community. In this sense a cognomen was like a nickname. For this reason, they were usually objective rather than subjective, concrete rather than abstract, and neutral or insulting rather than complimentary.
These characteristics are preserved in Nova Roma. It is important to understand that a cognomen is not a way for you to express your innermost thoughts or aspirations, or to boast about your wonderful qualities. It is just a name.
Inheritance
A Roman almost always takes his father's cognomen, especially if his father himself inherited the name from his father. Cases in which a cognomen may not be passed down from father to son are those where the cognomen is particularly closely associated with the father and would not be relevant to the son.
Agnomina are not usually inherited. Adoptive cognomina and matronymic cognomina are never inherited.
Ancient Republican Cognomina
A cognomen used in the ancient republic will normally be acceptable so long as it complies with the general characteristics noted above. Here is a list of some ancient republican cognomina with their meanings. Some cognomina were used especially by certain gentes: these are noted in the list, but they may also be used by members of other gentes.
Male Form | Female Form | Status | Meaning | Used especially by |
Aculeo | Aculeo | Prickly, unfriendly | ||
Agricola | Agricola | OVERUSED | Farmer | |
Agrippa | Agrippa | OVERUSED | Born feet first | Gens Menenia |
Ahala | Ahala | Armpit | Gens Servilia | |
Ahenobarbus | Ahenobarba | With a red beard | Gens Domitia | |
Albinus | Albina | Pale, white | Gens Postumia | |
Albus | Alba | White or fair-skinned | Gens Postumia | |
Ambustus | Ambusta | SUGGESTED | Burnt, scalded | Gens Fabia |
Annalis | Annalis | Relating to years | Gens Villia | |
Aquila | Aquila | OVERUSED | Eagle | |
Aquilinus | Aquilina | OVERUSED | Eagle-like | |
Arvina | Arvina | Fat, lard | Gens Cornelia | |
Asellio | Asellio | Keeper of donkeys | Gens Sempronia | |
Asina | Asina | Female donkey | Gens Cornelia | |
Atellus | Atella | Dark-haired or dark skinned | ||
Avitus | Avita | Grandfatherly | ||
Balbus | Balba | SUGGESTED | Stutterer | Gentes Acilia, Cornelia, Lucilia, Naevia, Octavia |
Barba | Barba | A beard | ||
Barbatus | Barbata | OVERUSED | Bearded | Gentes Cornelia, Horatia, Quinctia |
Bassus | Bassa | Plump | ||
Bestia | Bestia | Like an animal | Gens Calpurnia | |
Bibaculus | Bibacula | SUGGESTED | Drunkard | Gentes Furia, Sextia |
Bibulus | Bibula | SUGGESTED | Drunkard | Gentes Calpurnia, Publicia |
Blaesus | Blaesa | SUGGESTED | Someone who mispronounces words, slurs his speech, stammers, or lisps | Gens Sempronia |
Brocchus | Broccha | SUGGESTED | Toothy | |
Brutus | Bruta | Stupid, dull-witted | Gens Iunia | |
Bubulcus | Bubulca | SUGGESTED | Cattle-driver | Gens Iunia |
Bucco | Bucco | SUGGESTED | Fool, dolt | |
Bulbus | Bulba | SUGGESTED | Bulb, onion | Gens Atilia |
Buteo | Buteo | Buzzard | Gens Fabia | |
Caecus | Caeca | Blind | Gens Claudia | |
Caepio | Caepio | Onion-seller | Gens Servilia | |
Caesar | Caesar | DISCOURAGED | From archaic praenomen Caesar, perhaps meaning "hairy" | Gens Iulia |
Calidus | Calida | Hot-headed, rash | Gens Coelia | |
Calvinus | Calvina | From cognomen Calvus | Gentes Domitia, Veturia | |
Calvus | Calva | SUGGESTED | Bald | |
Camillus | Camilla | A child who helps during sacrifices | Gens Furia | |
Caninus | Canina | OVERUSED | Dog-like | Gens Acilia |
Canus | Cana | Golden-haired or grey-haired | ||
Capito | Capito | SUGGESTED | Big-headed | Gens Ateia |
Carbo | Carbo | Charcoal | Gens Papirii | |
Catilina | Catilina | Gens Sergia | ||
Cato | Cato | DISCOURAGED | Shrewd, prudent | Gentes Hostilia, Porcia |
Catulus | Catula | Puppy, whelp | Gens Lutatia | |
Celer | Celeris | Quick | ||
Celsus | Celsa | Tall | Gens Papia | |
Cethegus | Cethega | Gens Cornelia | ||
Cicero | Cicero | DISCOURAGED | Chick pea | Gens Tullia |
Cicurinus | Cicurina | Mild, gentle | Gens Veturia | |
Cilo | Cilo | SUGGESTED | Large forehead or large lips | Gens Flaminia |
Cincinnatus | Cincinnata | Curly-haired | Gens Quinctia | |
Cinna | Cinna | Gens Cornelia | ||
Cordus | Corda | Born late | ||
Cornicen | Cornicen | Military bugler | ||
Cornutus | Cornuta | Horned | Gens Caecilia, Sulpicia | |
Corvinus | Corvina | Crow-like | Gens Valeria | |
Corvus | Corva | OVERUSED | Crow | Gens Valeria |
Cossus | Cossa | From archaic praenomen Cossus | Gens Cornelia | |
Costa | Costa | A rib | Gens Pedania | |
Cotta | Cotta | Gens Aurelia | ||
Crassipes | Crassipes | SUGGESTED | Club-footed | Gens Furia |
Crassus | Crassa | OVERUSED | Fat | Gentes Claudia, Licinia, Otacilia, Veturia |
Crispinus | Crispina | Curly-haired | ||
Crispus | Crispa | Curly-haired | Gentes Sallustia, Vibia | |
Culleo | Culleo | SUGGESTED | Leather sack for carrying liquid | Gens Terentia |
Curio | Curio | SUGGESTED | Priest of a Curia | Gens Scribonia |
Cursor | Cursor | Runner, courier | Gens Papiria | |
Curvus | Curva | Stooping, bent | Gens Fulvia | |
Dentatus | Dentata | SUGGESTED | Toothy | |
Denter | Dentra | SUGGESTED | Toothy | Gens Caecilia |
Dento | Dento | SUGGESTED | Toothy | |
Dives | Dives | Rich, wealthy | Gens Licinia | |
Dolabella | Dolabella | Hatchet | Gens Cornelia | |
Dorsuo | Dorsuo | SUGGESTED | Large back | Gens Fabia |
Drusus | Drusa | Gens Livia | ||
Figulus | Figula | SUGGESTED | Potter | |
Fimbria | Fimbria | Edge of clothing, fringes | Gens Flavia | |
Flaccus | Flacca | SUGGESTED | Floppy ears | Gentes Aviania, Fulvia, Valeria |
Flavus | Flava | Blonde-haired | Gens Decimia | |
Florus | Flora | Light-coloured or blooming | Gens Aquilia | |
Fronto | Fronto | SUGGESTED | Prominent forehead | |
Fullo | Fullo | SUGGESTED | A fuller or launderer | Gens Apustia |
Fusus | Fusa | From archaic praenomen Fusus | Gens Furia | |
Galeo | Galeo | Helmet | ||
Gemellus | Gemella | A twin | Gentes Servilia, Veturia | |
Glabrio | Glabrio | A relative of Glaber | Gens Acilia | |
Gracchus | Graccha | Gens Sempronia | ||
Gurges | Gurges | Greedy, prodigal | Gens Fabia | |
Habitus | Habita | In good physical condition | Gens Cluentia | |
Helva | Helva | SUGGESTED | Dun-colored hair | Gens Aebutia |
Imperiosus | Imperiosa | DISCOURAGED | Domineering, dictatorial | Gens Manlia |
Iullus | Iulla | From archaic praenomen Iullus | Gens Iulia | |
Labeo | Labeo | Prominent lips | Gentes Antistia, Atinia, Fabia | |
Lactuca | Lactuca | Lettuce | Gens Valeria | |
Laenas | Laenas | A woolly cloak | Gens Popillia | |
Lanatus | Lanata | Wearing wool | Gens Menenia | |
Laevinus | Laevina | |||
Laterensis | Laterensis | Person from the hill-side | Gens Iuventia | |
Lentulus | Lentula | A bit slow | Gens Cornelia | |
Lepidus | Lepida | OVERUSED | Charming, amusing | Gens Aemilia |
Libo | Libo | SUGGESTED | Gens Marcia, Scribonia | |
Licinus | Licina | SUGGESTED | Spiky- or bristly-haired | Gens Mamilia |
Longus | Longa | SUGGESTED | Tall | Gentes Sempronia, Sulpicia |
Lucullus | Luculla | From lucus (grove) or Lucius (praenomen) | Gens Licinia | |
Lupus | Lupa | OVERUSED | Wolf | Gens Rutilia |
Lurco | Lurco | SUGGESTED | Gluttonous, greedy | |
Macer | Macra | SUGGESTED | Thin | Gens Licinia |
Macula | Macula | SUGGESTED | A spot or blemish | |
Malleolus | Malleola | Hammer | Gens Publicia | |
Mamercus | Mamerca | From rare praenomen Mamercus | Gens Aemilia | |
Marcellus | Marcella | OVERUSED | From praenomen Marcus | Gens Claudia |
Maro | Maro | SUGGESTED | Gens Vergilia | |
Merenda | Merenda | SUGGESTED | Light afternoon meal | Gentes Antonia, Cornelia |
Mergus | Merga | SUGGESTED | Sea-gull | |
Merula | Merula | Blackbird | Gens Cornelia | |
Messalla | Messalla | Gens Valeria | ||
Metellus | Metella | OVERUSED | Army follower | Gens Caecilia |
Murena | Murena | Eel | Gens Licinia | |
Mus | Mus | SUGGESTED | Mouse or rat | Gens Decia |
Musca | Musca | SUGGESTED | Fly | Gens Sempronia |
Nasica | Nasica | Big-nosed | Gens Sempronia | |
Naso | Naso | SUGGESTED | Big-nosed | Gens Ovidia |
Natta | Natta | SUGGESTED | An artisan | Gens Pinaria |
Nepos | Nepos | Grandchild | Gens Caecilia | |
Nero | Nero | From rare praenomen Nero ("strong") | Gens Claudia | |
Nerva | Nerva | Vigorous | Gens Cocceia, Licinia | |
Niger | Nigra | SUGGESTED | Black-skinned or black-haired | |
Novellus | Novella | New, new-fangled | Gens Gavilia | |
Ocella | Ocella | SUGGESTED | Small-eyed | Gens Livia |
Pacilus | Pacila | SUGGESTED | From archaic praenomen Pacilus | Gens Furia |
Paetus | Paeta | SUGGESTED | Squinty or blinking | Gens Aelia |
Pansa | Pansa | SUGGESTED | Splay-footed | Gens Vibia |
Papus | Papa | SUGGESTED | From rare praenomen Papus | Gens Aemilia |
Paterculus | Patercula | SUGGESTED | Little father | Gens Sulpicia |
Paullus | Paulla | Small | Gens Aemilia | |
Pavo | Pavo | Peacock | ||
Pera | Pera | SUGGESTED | Shoulder-bag | Gens Iunia |
Pictor | Pictrix | Painter | Gens Fabia | |
Piso | Piso | Mortar | Gens Calpurnia | |
Plancus | Planca | Flat-footed | Gens Munatia | |
Plautus | Plauta | SUGGESTED | Flat-footed | |
Poplicola | Poplicola | Friend of the people | Gens Valeria | |
Postumus | Postuma | Born after the father's death | Gens Curtia | |
Potitus | Potita | Probably derived from an archaic praenomen | Gens Valeria | |
Praeconinus | Praeconina | SUGGESTED | A relative of a herald | |
Praetextatus | Praetextata | Young (wearing the toga praetexta) | Gens Sulpicia | |
Priscus | Prisca | Very ancient | ||
Proculus | Procula | From rare praenomen Proculus, perhaps meaning "born during father's absence" | Gens Plautia | |
Publicola | Publicola | Variant of Poplicola | Gens Valeria | |
Pulcher | Pulchra | Attractive | Gens Claudia | |
Pullus | Pulla | SUGGESTED | Child | |
Pulvillus | Pulvilla | SUGGESTED | Small cushion | Gens Horatia |
Purpureo | Purpureo | SUGGESTED | Wearing purple or with a purplish complexion | |
Quadratus | Quadrata | Stocky, squarely built | ||
Ralla | Ralla | SUGGESTED | A tunic of fine fabric | Gens Marcia |
Regillus | Regilla | Prince | Gens Aemilia | |
Regulus | Regula | OVERUSED | Prince | Gens Atilia |
Rufus | Rufa | Reddish, Ginger-haired | ||
Ruga | Ruga | SUGGESTED | Wrinkly | |
Rullus | Rulla | SUGGESTED | Uncultivated, boorish | Gens Servilia |
Rutilus | Rutila | Reddish-gold hair | ||
Salinator | Salinatrix | Salt-harvester | Gens Livia | |
Saturninus | Saturnina | Dedicated to Saturnus | ||
Scaeva | Scaeva | Left-handed | Gens Iunia, Marcia | |
Scaevola | Scaevola | Left-handed | Gens Mucia | |
Scapula | Scapula | SUGGESTED | Shoulder-blade | Gens Quinctia |
Scaurus | Scaura | Lame, swollen-ankled | Gentes Aemilia, Aurelia | |
Scipio | Scipio | DISCOURAGED | Rod, staff | Gens Cornelii |
Scrofa | Scrofa | SUGGESTED | Sow | Gens Tremelia |
Seneca | Seneca | Elderly | Gens Annaea | |
Severus | Severa | OVERUSED | Strict, severe | |
Silanus | Silana | Nose, water-spout | Gens Iunia | |
Silo | Silo | SUGGESTED | Snub-nosed | Gens Sergia |
Silus | Sila | SUGGESTED | Snub-nosed | Gens Sergia |
Stolo | Stolo | SUGGESTED | Shoot of a plant | Gens Licinia |
Strabo | Strabo | Squinty | Gens Titia | |
Structus | Structa | Possibly derived from an archaic praenomen | Gens Servilia | |
Sulla | Sulla | DISCOURAGED | Gens Cornelia | |
Sura | Sura | SUGGESTED | Calf of the leg | |
Taurus | Taura | Bull | ||
Triarius | Triaria | OVERUSED | A type of soldier | Gens Valeria |
Trigeminus | Trigemina | A triplet | Gens Curiatia | |
Trio | Trio | SUGGESTED | One of the seven stars of the Plough / Big Dipper | Gens Lucretia |
Tubero | Tubero | SUGGESTED | Having a tumour or swelling | Gens Aelia, Iulia |
Tubertus | Tuberta | SUGGESTED | Having a tumour or swelling | Gens Postumia |
Tubulus | Tubula | SUGGESTED | Little tube | Gens Hostilia |
Tuditanus | Tuditana | SUGGESTED | Mallet | Gens Sempronia |
Tullus | Tulla | From rare praenomen Tullus | Gens Volcatia | |
Turdus | Turda | SUGGESTED | Thrush | Gens Papiria |
Varro | Varro | Block-head | Gens Terentia | |
Varus | Vara | Bow-legged | Gentes Atilia, Licinia, Quinctilia | |
Vatia | Vatia | Knock-kneed | Gens Servilia | |
Verres | Verres | Pig | Gens Cornelia | |
Vespillo | Vespillo | Person employed to bury people too poor for a funeral | Gens Lucretia | |
Vetus | Vetus | Old | Gens Antistia | |
Vitulus | Vitula | Calf or young cow | Gentes Mamilia, Pomponia | |
Volusus | Volusa | From rare praenomen Volusus | Gens Valeria |
Geographical Cognomina
Some people have cognomina which refer to the place where they come from, whether a city (e.g. Collatinus, "man from Collatia"), a region (e.g. Campanus, "man from Campania"), or a tribe (e.g. Sabinus, "man of the Sabines"). Names referring to whole countries or provinces (e.g. Hispanus, "man from Hispania") are not normally allowed because they are not distinctive enough: if everyone in Hispania were called Hispanus, it would be very confusing! But such names may be appropriate if you live away from your native country (e.g. a citizen from Hispania living in America might be called Hispanus).
These geographical cognomina should be distinguished from honorific cognomina like Germanicus or Britannicus. These names do not refer to a place of origin but to a military achievement. If someone is called Britannicus it does not mean he is from Britain, it means he won a great victory against the Britons. Names like this are not allowed, for obvious reasons.
If you would like to use a geographical cognomen, the Censores will work with you to find an appropriate one.
Occupational Cognomina
Another type of cognomen in ancient times was one which referred to the person's job or occupation (e.g. Pictor, "painter"; Caprarius, "goat-herd"). If you would like to use an occupational cognomen, the Censores will work with you to find an appropriate one.
Latinising your own name
Often when a foreigner became a Roman citizen in ancient times he would keep his old name as a cognomen, adjusting it to make it easier for Latin-speakers to say and giving it a Latin ending. This is also a common option in Nova Roma.
If you choose this option, you can use either your first name or your surname, or both. If you use both, they will be put in reverse order. For example, Robert Grant would take the cognomina Grandis Robertus. This is because in a Roman name the second cognomen is more individual than the first.
If you would like to use a Latin form of your own name, the Censores will work with you to find the appropriate form. Here are the Latin forms of some common names:
The Table of Latinised Names is under revision.
Other Latin Words
Many ordinary Latin nouns and adjectives can be used as cognomina. If there is some particular Latin word you would like to use as a cognomen, or if you would like a cognomen with a particular meaning, the Censores will work with you to find an appropriate name.
Multiple Cognomina
In some cases it may be appropriate to have more than one cognomen. This is normally only allowed where you use a Latinised form of your own name, but may also be appropriate if you are joining a family which contains a large number of people who all have the same nomen and cognomen. If you think you have a good reason to want more than one cognomen, the Censores will discuss it with you.
Special Cognomina
There are several types of cognomen which serve a particular purpose and which are therefore not available for new citizens.
Adoptive Cognomina
When a Roman citizen is adopted by another, he takes the name of his adoptive father, but adds a special cognomen to indicate his former identity. This cognomen is formed from his old nomen, with the -ius ending replaced with an -ianus ending.
For example, when L. Aemilius Paullus was adopted by P. Cornelius Scipio he became P. Cornelius Scipio Aemilianus.
Matronymic Cognomina
In very rare cases a Roman might use an extra cognomen formed from his mother's nomen. The -ia ending was replaced with an -ianus ending or an -inus ending.
For example, M. Porcius Cato had one son by his first wife Licinia, and another son by his second wife Salonia. Each son was called M. Porcius Cato. To tell them apart, people called them M. Porcius Cato Licinianus and M. Porcius Cato Salonianus.
Agnomina
Sometimes very eminent Romans were given honorific cognomina in recognition of their great achievements. These were the exception to the general rule that cognomina were not complimentary. Such honorific cognomina are called agnomina. Obviously a person cannot give himself an agnomen: it is always given by others.
An agnomen may refer to a victory over a particular enemy people (e.g. Britannicus, "victor over the Britons") or in a particular place (e.g. Africanus, "victor in Africa"), a particular virtue (e.g. Pius, "dutiful"; Sapiens, "prudent"), or general preeminence (e.g. Magnus, "great"; Maximus, "very great").
Other Elements
Although the tria nomina are the core of a Roman name, there are times when a Roman might include other elements in his name. These are not really part of his name as such but are additional pieces of information about him and his place in the community.
Filiation
After a person's nomen and before his cognomen a Roman may include the praenomen of his father and, sometimes, his father's father. This is done in the following way:
M. Tullius M. f. M. n. Cicero
This means "Marcus Tullius Marci filius Marci nepos Cicero", or "Marcus Tullius, son of Marcus, grandson of Marcus, Cicero".
Tribe
Every Roman citizen is a member of a tribe. These are not ethnic groups but social units, communities within the community, and voting-blocks. A Roman is born into his father's tribe. There are thirty-five tribes, each with a standard abbreviation:
The tribe is inserted between the nomen and the cognomen, e.g. M. Tullius Cor. Cicero (meaning "Marcus Tullius Cornelia tribu Cicero", or "Marcus Tullius, of the Cornelian tribe, Cicero").
Often the tribe is included together with the filiation, in which case it appears after the filiation and before the cognomen, thus:
M. Tullius M. f. M. n. Cor. Cicero
Usage
This section contains general guidance on how to use Roman names. It must be stressed that these are not firm legalistic rules. A name is a device which allows one person to talk about another person so that everyone knows who he is talking about. Any rule which makes it unclear who is being referred to would be self-defeating, and all the general rules explained below should be ignored if following them would lead to confusion.
How Many Names?
As a general rule, the more formal the context, the more names are used. The use of all three (or more) names is very formal, and should be rare. Calling someone M. Tullius Cicero is roughly equivalent to calling someone Mr Robert James Grant, Esquire.
Two names are normally enough to make it clear who you are talking to or about. Using two names is formal and polite. Calling someone M. Tullius is roughly equivalent to calling someone Robert Grant or Mr Grant. When you mention someone for the first time in a speech or a letter, or when greeting someone, it is common to use two names.
Use of one name is relatively relaxed and informal. If you already in the middle of a conversation with someone, or in the middle of talking about someone, you might well call him by just one name, especially if you know him reasonably well. Calling someone Cicero is roughly equivalent to calling someone Robert. But in formal situations or when first mentioning someone, using only one name may be over-familiar and could be impolite.
Which Names?
Which names you call someone by depends partly on how many names you are using.
Two Names
When calling someone by two names, which names you use depends on the status of the person you're naming. If the person is a nobilis, it is proper to call him by his praenomen and cognomen, e.g. P. Scipio. If he is a novus homo, one would normally call him by his praenomen and nomen, e.g. M. Tullius. Most people in Nova Roma are novi homines, so most people are normally called by their praenomen and nomen. If you accidentally call a nobilis as if he were a novus homo, he will probably not take offence, especially if you apologise for your mistake; but if in doubt, you can always ask.
One may, of course, flatter or praise a novus homo by naming him with his praenomen and cognomen as if he were a nobilis; but you should not do this too often, especially as it could cause resentment among the true nobiles.
Some people adopt a style which was fashionable in the last generation of the ancient republic and call everyone, both nobilis and novus homo, by nomen and cognomen, e.g. Cornelius Scipio, Tullius Cicero. There is nothing strictly wrong with this, but it is not characteristic of the ancient republic and is not encouraged.
One Name
When calling someone by only one name, it is normal and polite to use the cognomen. A nobilis should always be called by his cognomen. A novus homo can be called by his nomen: this is not strictly impolite, but it is at best neutral and may also make it unclear who you are talking about.
Where a person has more than one cognomen, you should normally use the first one. Calling someone by his agnomen, if he has one, is of course particularly complimentary. You should only call someone by his adoptive if you want to draw attention to his pre-adoption family and identity: this is not necessarily polite or impolite, but will depend on the context. Similarly caling someone by his matronymic cognomen will draw attention to his mother's identity and family.
Do not fall into the trap of calling someone routinely by his adoptive cognomen. This is often tempting, because it is an easy way of distinguishing between the adopted child and the adoptive father, but it is an un-Roman habit. To a Roman, an adopted child became, to all intents and purposes, the child of the adoptive parents, and one should normally ignore his adoptive cognomen when naming him.
Praenomen Only
The praenomen is essentially a private name, for use within the family. You should not call a Roman by just his praenomen unless he is your close relative or very, very close friend. Even spouses do not generally call each other by their praenomina alone - they generally use nomina or cognomina.
Latin Vocatives
When you call someone by name, you change the ending of the name to indicate that you are talking to the person, not about him. As a general rule, names ending in -us take an ending -e (e.g. Brutus -> Brute), though names ending in -ius turn to -i (e.g. Tullius -> Tulli). Names ending in -a do not usually change. Names with other endings do not usually change.
You may notice some people using vocative endings when they are talking about someone in the third person (e.g. "I was talking to Brute yesterday"). Don't be confused - you are right, they are wrong.
Addresses Other Than Names
Much more than in some modern societies, Romans address each other using labels other than names, or combine names with other terms. What follows is a very brief overview.
Dominus & Domina
Some modern Latin-speakers use "dominus" and "domina" as equivalents of English "Mr" and "Miss" or "Mrs". This is strongly discouraged. "Dominus" means "lord" or "master", and addressing someone in this way is very servile and grovelling.
An exception is that lovers sometimes call each other "dominus" and "domina", though usually only in the bedroom.
Titles
Though Romans do not generally go in for titles in a big way, it is not uncommon to call a Consul by the title "Consul", for example, especially when speaking in a political context or discussing business relevant to the office. Likewise one may call one's patron by the title "patronus". But titles are not by any means compulsory, and there is nothing at all rude in calling a magistrate straightforwardly by his name.
Relatives
As well as calling each other by name, relatives commonly talk to or about each other by reference to their relationship, e.g. pater (father), soror (sister), patruus (uncle), &c. These terms are often combined with terms of endearment (see below). As noted above, close relatives might call each other by their praenomina.
Spouses and Lovers
As was mentioned above, spouses and lovers generally call each other by cognomen rather than praenomen. Occasionally they called each other vir (husband) and uxor (wife), but more commonly they used terms of endearment (see below).
Friends and Acquaintances
Most of the time people who know each other but are not especially close call each other by name, sometimes with "mi" (see below). Sometimes they will use brief descriptions, e.g. iuvenis (young man), amicus (friend), senex (old man). Depending on the relationship between the people concerned, they may use terms of endearment or even insults.
Strangers
There is no direct Roman equivalent of "sir" or "madam". If you meet someone whose name you don't know, it is normal and not at all rude to say something like "petasate" ("you with the hat") or "senex" ("old man") or "viator" ("traveller"). Very often one might say "quiquis es" ("whoever you are").
Generally, though, unless you are merely asking the time of day, the best tactic is to try to find out the person's name by saying something like "adulescens, dic mihi nomen tuum, quaeso" ("young man, please tell me your name") or "o qui vocaris?" ("O how are you called?").
Terms of Endearment and Esteem
Romans have always been very inventive with terms of endearment. One very common one is "carissimus", often combined with a name, e.g. "salve Brute carissime" ("hello my dear Brutus"), "salve soror carissima" ("hello dear sister"). Others include "dulcis" ("sweet"), "inclitus" ("famous"), "magnus" ("great"), "optimus" ("excellent"), "fortissimus" ("very strong"). This should be enough to give you the general idea.
"Mi" and "O"
"Mi" (masculine) and "mea" (feminine) mean "my". They are very commonly attached to names or other terms of endearment in conversations between friends or well-meaning acquaintances, e.g. "salve mi frater" ("hello my brother"), "salve mea Cornelia" ("hello my Cornelia"). It is especially common in letters, not so much in oral conversation.
"O" is also not uncommon. It has the general effect of making an address to someone more emotional, emphatic, or poetic. E.g. "o optime Brute" ("O excellent Brutus").