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The index of all marriages registered during the first three months of 1975 constitutes a random sample of the breeding population of England and Wales. Among listings there are 32,457 different surnames. The coefficient of relationship by isonymy from these is 0.0004823. The rarer the surname, the more frequently, on average, it recurs in the same district. Rare surnames (those occurring 1 to 6 per 100,000) account for 93% of different surnames but for only 0.5% of the coefficient of relationship. Nevertheless rare surnames are less polyphyletic and show more localization than common surnames. Regional studies of isonymy are therefore most useful if the contribution of different surnames, categorized according to their national frequencies, are separately evaluated.