Given this basic definition, further criteria are needed to establish a dividing line between postlexical clitics and morphological affixes, since both are characterized by a lack of prosodic autonomy. There is no natural, clear-cut boundary between the two categories (since from a historical point of view, a given form can move gradually from one to the other by morphologization). However, by identifying clusters of observable properties that are associated with core examples of clitics on the one hand, and core examples of affixes on the other, one can pick out a battery of tests that provide an empirical foundation for a clitic/affix distinction.