This paper argues that Czech verbal prefixes alternate between two states, roughly corresponding to the traditional notions ‘free’ and ‘bound’. The distinction, however, is not reflected in the separability of the prefix and the verb; it is reflected in vowel length. Main evidence for the claim is drawn from the way vowel length of adpositions is treated Czech internally and from a comparison to Norwegian. Theoretically, we implement the alternation as a phrasal movement of the prefix from a VP internal position (where the prefix behaves as bound) to a VP external position, drawing on Taraldsen’s (2000) proposal for Norwegian and Svenonius’ (2004) account of prefixation in Slavic.