Within-lineage repeated-pair isonymy is calculated as RPW = sigma [Sij(Sij - 1)]/sigma [Si(Si - 1)], in which Sij is the number of a combination of two surnames in marriages (for total RPW, in both orders, brides first and also grooms first) and Si is the corresponding number of individuals of each surname (for total RPW of both sexes). In St. Ouen the 3479 native Jersey marriages between 1634 and 1914 yield a mean total RPW of 0.03314, which can be compared with a random isonymy (Ir) of 0.02205 and a marital isonymy (Im) of 0.04942. In the most recent time periods (1830-1914) there was an increase in RPW, but the increase is small if all marriages are considered rather than only native Jersey ones. In the Scilly Isles 2827 marriages between 1725 and 1975 yield a mean total RPW of 0.05127, which compares with a mean Ir of 0.01960 and an Im of 0.05238. In each kind of isonymy there is a decline over 1850-1975 caused by increased migration to the Scilly Isles. In the Scilly Isles RPW consistently approximately equals Im, so the nonrandom component of inbreeding is fully accounted for by factors that affect marital choice in the whole lineage. In St. Ouen, however, RPW is intermediate in value between Ir and Im, so the nonrandom component of inbreeding is only partly a lineage phenomenon and is also affected by other (individual) factors in choice of spouses.