Counting Y-DNA STR differences is a poor discriminator for members of the same family. Unless a very large number of markers are used, matching cannot be used with a statistically significant degree of accuracy to establish whether someone is more closely related even to second cousins. However there are other methods that may be used to distinguish degrees of relatedness. We present a simple and useful test which involves finding a “segmenting marker” which can establish relative consanguinity with some accuracy, and we give a real-life example of its use to show from which of two 17th Century brothers a man of partly unknown origins descended.