This paper analyzes Czech double diminutives ending in '-ecek' created by the recursive application of the suffix '-ek' whose initial vowel alternates with zero. Diachronically speaking, these diminutives display both patterns of V-zero alternations found in Slavic languages: in OCz they follow the Havlik pattern, where alternants are in complementary distribution ('domocek'), while in MoCz they follow the Lower pattern, where strong alternants (i.e. vowels) are always preceded by strong alternants ('domecek'). The analysis of the Havlik-to-Lower change presented follows Rubach's (1984) classical analysis where the Lower pattern is derived from the cyclic application of the Lower rule which means that only the Lower pattern has internal phase structure. The author argues that in the Lower pattern, all floating vowels in a row (except the final one) vocalize, because each is immediately followed by an empty nucleus which stands at the phase boundary. Furthermore, phasehood is a lexical property, i.e. a property of a particular lexical item, namely the diminutve suffix '-ek'. From this perspective, the Havlik-to-Lower change consists in a change in the properties of the lexicon: only in MoCz is the suffix '-ek' lexically specified as a phase-trigger, in OCz it did not trigger any phase.