TRIGENIA. - This is a particular kind of affinity (v.), currently considered only by the canon law of some Greek-rite communities.
In all the Eastern rites, affinity exists between each spouse and the relatives of the other, and it constitutes for Catholics a diriment impediment to marriage up to and including the fourth degree: this is, in modification of what is stated in the article Affinity, the law in force since 2 May 1949 (motu proprio Crebrea allata, 22 Feb. 1949, cann. 67 § 1, n. 1, 68 § 1). In some rites, moreover, the bond existing between the relatives of one spouse and the relatives of the other is also considered as affinity: this too, at present, constitutes a matrimonial impediment up to and including the fourth degree; and the degrees of this are calculated by adding the respective degrees of relationship of each of the two affines with the spouse to whom they are related (motu proprio cit., cann. 67 § 2, 2). Both these kinds of affinity are called affinity from dignity (ἐκ ἀγαγείας), inasmuch as in it two stocks are involved, namely that of the relatives of the husband and that of the relatives of the wife.
Trigynia can occur in two cases: 1) when a person contracts several marriages in succession; 2) when two or more married persons are related to one another (motu proprio cit., cann. 68, § 3, n. 1).
In the first case, trigynia exists between those who have contracted marriage with the same person (but since they are of the same sex, trigynia in this case is of little importance, even if they are all alive); and also between each of them and the relatives of the others (motu proprio cit., cann. 68 § 3, n. 2); and in some communities also between the relatives of each of them and the relatives of the others (motu proprio cit., cann. 68 § 3, n. 4).
In the diagram above, therefore, there will be trigynia between B and C, between B and e-l, and between C and a-d; moreover, in communities where trigynia is more extensive, a-d will also be trigynous affines of e-l.
In the second case, trigynia exists between each spouse and those who, because of another marriage, have affinity from dignity with the other spouse; and also, in some communities, between the relatives of that spouse and these affines (can. 68 § 3, nn. 2 and 4).
Trigynia will therefore exist, on the basis of these diagrams, between A and D, d, e, f, as well as between D and A, a, b; in communities that consider trigynia more extensively, it will also exist between a, b and d, e, f.
Where it exists, trigynia is a matrimonial impediment only in the first degree.
The computation of the degrees of trigynia is made, in ordinary cases, by counting as the degree of trigynia of each person toward a spouse the same degree in which he is an affine by dignity toward the other spouse because of another marriage; in cases of trigynia extended to the relatives of the spouse, the respective degrees of affinity from dignity and of relationship are added together to obtain the degree of affinity from trigynia (motu proprio cit., can. 68 § 3, nn. 3-4). No rule is established for the calculation of the degree of trigynia between those who have contracted marriage with the same person, since this trigynia is not of concern as a matrimonial impediment: but it is evident that, being this the closest possible degree of trigynia, it must be considered of the first degree.
The name affinity from trigynia (ἐκ τρίγενελας) derives from the fact that in every case of trigynia three stocks or families are involved, namely those of each of the contracting parties of the two marriages that give rise to trigynia: they are three, not four, because either one contracting party is common to both marriages, or a contracting party of one is related to a contracting party of the other and they therefore belong to a single stock.
Affinity from trigynia was also known to Latin canon law, at least in the 11th-12th centuries; and naturally it also constituted a kind of puzzle in the West. Hence Innocent III, with can. 50 of the Second Lateran Council (1216), abolished in the Latin Church affinity from trigynia, as well as the second kind of affinity from dignity.