Using lists of names of male personal telephone subscribers, isonymy was calculated within and between 29 contiguous areas in a north-to-south line extending 210 km south of Grindsted, Denmark. Each area shared some surname(s) with every other area. Isonymy was high across both the present and past borders of Denmark with Germany and was consistently lower in areas beyond 160 km south of Grindsted. Relative isonymy between areas was also smaller on average south of the present border than north of it, and smaller still for pairs of areas spanning the border. This is partly accounted for by decreases in isonymy with distance, but the slope of the logistic regression on distance is greater for the northern moiety than the southern one. Most of these findings can be traced to the influence of common surnames ending in ‘sen’, the distribution of which tends to correlate highly with isonymy. Such surnames tend to be of recent origin and to be very frequent and hence highly polygenic. Thus much of the heterogeneity is explained by surname history rather than genetic heterogeneity.