SYNOPSIS. This essay presents a method for measuring the degree to which one biological outline form lies in between two others. The procedure does not measure forms separately, but rather compares pairs of tensors expressing D'Arcy Thompson's “Cartesian transformations” according to the biorthogonal formalism of Bookstein. In analogy with conventional methods, betweenness is computed as a similarity score, the cosine of a non- Euclidean angle between the tensors. The new quantities, size-betweenness and shapebetweenness, enable comparisons of form series against a priori orderings intra- and interspecifically