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Under certain assumptions the commonality of surnames in two lists of names yields a coefficient of relationship by isonymy (Ri) of the populations from which the samples were drawn. The 6,142 principals to marriages between 1753 and 1950 in eight Otmoor Church of England parishes permitted many estimates of Ri between pairs of places of residence and between the sexes within single parishes. By calculating Ri of the same pair of communities in different ways, reliabilities of 20-70% are demonstrated. Ri for 1753-1850, for 1851-1950 and for 1976 are positively correlated with each other but not highly so. The Ri within single parishes is nearly an order of magnitude higher than that between parishes and that between pairs of the 8 parishes is higher on average than those with other places. Ri between parishes tended to increase from about 1800 to about 1900 and to decrease by 1976. Ri is positively correlated with marital migration and tends to be negatively correlated with distance between parishes.