A Roman marriage was called Justae Nuptiae, Justum Matrimonium, Legitimum Matrimonium, as being conformable to Jus Civile or to Roman Law. A marriage was either Cum conventione uxoris in manum viri, or it was without this conventio. In both cases there must be connubium between the parties, and consent: the male must also be pubes, and the woman viri potens. The legal consequences as to the power of the father over his children were the same in both. Opposed to the Legitimum Matrimonium was the Matrimonium Juris Gentium.

A Roman marriage may be viewed, First with reference to the conditions required for a Justum Matrimonium; Secondly, with reference to the forms of the marriage; Thirdly, with reference to its legal consequences.

Unless there was connubium there could be no Roman marriage. Connubium is defined by Ulpian (Frag. v.3) to be "uxoris jure ducendae facultas", or the faculty by which a man may make a woman his lawful wife. But in truth this is no definition at all, nor does it give any information. Connubium is merely a term which comprehends all the conditions of a legal marriage. Accordingly, the term is explained by particular instances: "Roman men citizens," says Ulpian, "have connubium with Roman women citizens (Romanae cives); but with Latinae and Peregrinae only in those cases where it has been permitted. With slaves there is no connubium."

Sometimes connubium, that is the faculty of contracting a Roman marriage, is viewed with reference to one of its most important consequences, namely, the Patria Potestas: "for," says Gaius, "since it is the effect of Connubium that the children follow the condition of their father, it results that when Connubium exists, the children are not only Roman citizens, but are also in the power of their father." Generally, it may be stated that there was only connubium between Roman citizens: the cases in which it at any time existed between parties, not both Roman citizens, were exceptions to the general rule. Originally, or at least at one period of the Republic, there was no Connubium between the Patricians and the Plebeians; but this was altered by the Lex Canuleia which allowed Connubium between persons of those two classes.

There was no connubium between many persons with respect to one another, who had severally connubium with respect to other persons. Thus there were various degrees of consanguinity within which there was no connubium. There was no connubium between parent and child, whether the relation was natural or by adoption; and a man could not marry an adopted daughter or granddaughter, even after he had emancipated her. There was no connubium between brothers and sisters, whether of the whole or of the half blood; but a man might marry a sister by adoption after her emancipation, or after his own emancipation. It became legal to marry a brother's daughter after Claudius had set the example by marrying Agrippina; but the rule was not carried further than the example, and in the time of Gaius it remained unlawful for a man to marry his sister's daughter (Gaius, i.62; Tacit. Ann. xii.5; Sueton. Claud. 26).

There was no connubium also between persons within certain relations of affinity, as between a man and his socrus, nurus, privigna, and noverca.

Any illegal union of a male and female, though affecting to be, was not a marriage: the man had no legal wife, and the children had no legal father; consequently they were not in the power of their reputed father. These restrictions as to marriage were not founded on any enactments; they were a part of that large mass of Roman law which belongs to Jus Moribus Constitutum. The marriage of Domitius, afterwards the emperor Nero, with Octavia the daughter of Claudius, seems at first sight somewhat irregular. Nero was adopted by Claudius by a Lex Curiata (Tacit. Ann. xii.26), but he was already his son-in-law; at least the sponsalia are mentioned before the adoption (Tacit. Ann. xii.9). There seems to be no rule of law which would prevent a man from adopting his son-in-law; though if the adoption took place before the marriage, it would be illegal, as stated by Gaius.

Persons who had certain bodily imperfections, as eunuchs, and others who from any cause could never attain to puberty, could not contract marriage; for though pubertas was in course of time fixed at a positive age [IMPUBES], yet as the foundation of the notion of pubertas was physical capacity for sexual intercourse, there could be no pubertas if there was a physical incapacity.

The essence of marriage was consent, and the consent, says Ulpian, "both of those who come together, and of those in whose power they are;" and "marriage is not effected by sexual union, but by consent." Those then who were not sui juris, had not, strictly speaking, connubium, or the "uxoris jure ducendae facultas"; though in another sense, they had connubium by virtue of the consent of those in whose power they were, if there was no other impediment (Dig. 23 tit.1 s11-13).

The Lex Julia et Papia Poppaea placed certain restrictions on marriage as to the parties between whom it could take place. [JULIA ET PAPIA POPPAEA; INFAMIA.]

A man could only have one lawful wife at a time; and consequently if he were married, and divorced his wife, a second marriage would be no marriage, unless the divorce were effectual.

The marriage Cum conventione in manum differed from that Sine conventione, in the relationship which it effected between the husband and the wife; the marriage Cum conventione was a necessary condition to make a woman a materfamilias. By the marriage Cum conventione, the wife passed into the familia of her husband, and was to him in the relation of a daughter, or as it was expressed, "in manum convenit" (Cic. Top. 3; filiae loco est, Gaius, ii.159). In the marriage Sine conventione, the wife's relation to her own familia remained as before, and she was merely Uxor. "Uxor," says Cicero (Top. 3), "is a genus of which there are two species; one is materfamilias, 'quae in manum convenit; the other is uxor only." Accordingly a materfamilias is a wife who is in manu, and in the familia of her husband, and consequently one of his sui heredes; or in the manus of him in whose power her husband is. A wife not in manu was not a member of her husband's familia, and therefore the term could not apply to her. Gellius (xviii.6) also states that this was the old meaning of materfamilias. Matrona was properly a wife not in manu, and equivalent to Cicero's "tantummodo uxor"; and she was called matrona before she had any children. But these words are not always used in these their original and proper meanings (see Ulp. Frag. iv).

No forms were requisite in marriage; the best evidence of marriage was cohabitation matrimonii causa. The matrimonii causa might be proved by various kinds of evidence. A marriage Cum conventione might be effected by Usus, Farreum, and Coemptio.

If a woman lived with a man for a whole year as his wife, she became in manu viri by virtue of this matrimonial cohabitation. The consent to live together as man and wife was the marriage; the usus for a year had the manus as the result; and this was by analogy to Usucapion of movables generally, in which usus for one year gave ownership. The Law of the Twelve Tables provided that if a woman did not wish to come into the manus of her husband in this manner, she should absent herself from him annually for three nights (trinoctium) and so break the usus of the year (Gell. iii.2; Gaius, i.111). The Twelve Tables probably did not introduce the usus in the case of a woman cohabiting with a man matrimonii causa, any more than they probably did in the case of other things; but as in the case of other things, they fixed the time within which the usus should have its full effect, so they established a positive rule as to what time should be a sufficient interruption of usus in the case of matrimonial cohabitation, and such a positive rule was obviously necessary in order to determine what should be a legal interpretation of usus.