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Our genealogy program's kinship report shows categories called Civil, which has Roman Numerals, and Cannon, with Arabic numerals. What do cannon
A. Canon (correctly spelled with one n) and civil have to do with degrees of relationship between relatives—the terms stand for different methods to calculate the degrees.
Canon law records the number of steps back to two relatives' common ancestor. For example, your first cousin is two steps to the ancestor you two share (your grandparent)—so the canon number is 2.
Civil relationships give the total number of steps from one relative to the other. In the case of your first cousin, it's two steps from you to your grandparent and two more from your grandparent to your first cousin. That makes the civil relationship IV (as you've noted, it's customarily shown in Roman numerals).