Cognation

Capitis deminutio extinguishes rights by agnation, while it leaves unaffected rights by cognation, because civil changes can take away rights belonging to civil law (jus civile), but not rights belonging to natural law (jus naturale).

Sed agnationis quidem ius capitis deminutione perimitur, cognationis uero ius eo modo non commutatur, quia ciuilis ratio ciuilia quidem iura corrumpere potest, naturalia uero non potest.

When Justinian consolidated the law of intestate succession and made the right of succession depend on cognation instead of agnation, he made a corresponding change in the obligation of guardianship, which henceforth devolved on cognates instead of agnates, women as formerly, with the exception of mothers and grandmothers, being excluded from the office, Nov.

Cognatic relationship is simply the conception of kinship familiar to modern ideas;

The Prætors early laid hold on Cognation as the natural form of kinship, and spared no pains in purifying their system from the older conception.

or, if we take the strict technical meaning of the word in Roman law, they are all who trace their blood to the legitimate marriage of a common pair.