Evidence from Italian conjugation shows that the structure of a verbparadigm can be characterised formally in terms of a distribution of slotsinto a number of equivalence classes, or set partition, where each equivalenceclass is associated with a distinct Aronovian basic stem type. A few set partitionsaccount for the structure of all Italian verb paradigms, whether regular orless regular. This well-behaved family of distributions tightly constrainsstem allomorphy at an appropriate level of abstraction, independently of whetherthe origin of allomorphy is morpho-phonological or purely morphological, showingthe superiority of the obtained generalisations over syntagmatic accounts.In the paper, we illustrate some formal properties of the discovered partitions,and emphasise interesting connections with morphological productivity, historicalchange and language learning.