It has been customary to regard morphological irregularity as an accident of language change and accordingly its eventual disappearance as only a matter of time. This study undertakes a fundamental re-evaluation of that assumption. On the basis of a contrastive analysis of ten highly frequent verbs in ten Germanic languages it shows that irregularity is in fact consistently generated in an astonishingly systematic way. In the course of the discussion it transpires that irregularity and brevity of expression are closely associated. The detailed description of irregularity principles is followed by a theoretical section. Here, the negative concept of irregularity is replaced by that of differentiation, which is shown to have a positive function in connection with high token frequency because it allows for formal brevity without any risk of syncretism. Against this background total suppletion represents the ideal combination of maximal differentiation and brevity of expression.