One of the most common ways of morphological marking is affixation, morphemes are classified according to their position. In languages with affixal morphology, suffixes and prefixes are the most common types of affixes. Despite several proposals, it has been impossible to identify solid generalisations about the behaviour of prefixes, in opposition to suffixes. This article argues that the reason is that our traditional definitions of suffix and prefix are based on pre-theoretical, surface criteria that have been given up in other (...) areas of linguistics: defining a morpheme as a prefix does not tell us anything about its grammatical nature, as that label does not take into consideration the structural configuration underlying the morpheme. Once the structural configuration is taken into account, solid generalisations begin to emerge. The article illustrates the advantages of this approach through a study of the interaction between vowel harmony and affixes. (shrink)