We specify a typology for the extreme of inflectional morphology, namely suppletion (as in go ~ went). This is an unusual enterprise within typology, and it requires a ‘canonical’ approach. That is, we define the canonical or best instance, through a set of converging criteria, and use this point in theoretical space to locate the various occurring types. Thus the criteria establish the dimensions along which we find the specific instances of suppletion, allowing us to calibrate examples out from the canonical. The criteria fall into two main areas, those internal to the lexeme and those external to it. Moreover, we find interactions with other morphological phenomena, and discuss four of them: syncretism, periphrasis, overdifferentiation and reduplication. These remarkable instances of suppletion, particularly when in interaction with other phenomena, extend the boundary of the notion ‘possible word’. Besides laying out the possibilities for the specific phenomenon of suppletion, we show how a canonical approach allows us to make progress in typology, even in the most challenging areas.