All changes to the internal structure of phonological segments arise from combinations of two set-theoretic operations: feature deletion by set subtraction and feature insertion by unification. Once stated, the idea seems trivial—after all, it’s obvious that you can make a d into a t by removing +VOICED and adding −VOICED. However, we’ll see that some subtleties are required in the chain of reasoning that will lead us to a full treatment of the complex process of Hungarian voicing assimilation. This example illustrates how the problem of modelling the full range of phonological phenomena can be fruitfully decomposed.