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There have been several attempts to use surnames to give some indication of population change over time or distance. An extension of isonymy theory provides, with certain assumptions, a coefficient of relationship between two populations.The present study addresses the validation of this coefficient of relationship using data from the Orkneys. The coefficient of relationship between two samples was calculated and compared with the kinship coefficients obtained from pedigree analysis, and the observed relationships compared with the surname pattern for 150 individuals. There is an apparent sevenfold overestimate of mean kinship when isonymy is used to estimate it. However, the observed number of isonymous pairs of relatives is 4.0 which corresponds quite closely to that expected, 4.39. The major source of isonymic overestimation of relationship is related to the earlier patronymic system of surnames in the Orkneys.