I specify a typology for the extreme of inflectional morphology, namely suppletion (as in go ϳ went). This is an unusual enterprise within typology, and it requires a 'canonical' approach. That is, I 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 specific instances of suppletion are found, allowing me 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, I 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, I show how a canonical approach allows progress in typology, even in the most challenging areas.* INTRODUCTION. If the examples were not so familiar, linguists would look harder at instances of suppletion, like go ϳ went and Russian rebenok 'child' ϳ deti 'children'. Such items are significant for our view of morphology (as Carstairs-McCarthy (1994: 4410) points out). They are also a limiting case for the notion POSSIBLE WORD, and as such they constitute a real challenge to the typologist. There have been several accounts of suppletion, starting with Osthoff 1899; an annotated bibliography of this work with over seventy items is provided by Chumakina (2004). However, the definitions and surveys proposed do not yet constitute a full typology. The focus of this article is therefore a specification of the criteria for a typology of suppletion. To propose a typology in this apparently hostile terrain means adopting a CANONICAL APPROACH, one that defines a principled point in the theoretical space and calibrates outwards from it.I start by outlining the basic notions needed and then consider the place of suppletion. The heart of the article is the criteria for the typology, which divide naturally into two main types, those internal to the lexeme and those external to the lexeme. These are taken up in turn. In addition, I treat the question of interactions with other morphological phenomena, which extend further the notion possible word, and finally address the interesting issue of the frequency of suppletive items.