|Abstract:||Syncope and apocope of post-tonic vowels in the early, pre-literary, period of the language made a major contribution to the characteristic phonological structure of the Catalan lexicon. The process eliminated the majority of cases of the post-tonic central vowel ([ə]) deriving from /e/, /i/, /o/, /u/ of Vulgar Latin, and thereby vastly reduced the number of syllables in lexical entries. For example, the process converted almost all masculine paroxytones to oxytones, and almost all proparoxytones to paroxytone. Most scholars have regarded syncope and apocope as independent changes, overlooking the fact that from this perspective both processes should be able to apply, provided syllable constraints permit. In fact, no more than one post-tonic vowel is deleted in any word. I suggest that syncope and apocope of post-tonic vowels both reflect two preferences that overcame faithfulness to the syllabic and prosodic structures inherited from late Latin. The first was a preference for trochaic over dactylic feet (reflected here in the constraint HEAD FOOT-RIGHT); the second was the preference not to realize the least distinct vowel of the inventory /ə/ (reflected in the constraint *SCHWA). The effect of these two preferences (constraints) was countered by a faithfulness constraint (MAX PROSODIC HEAD) that favoured retaining a vowel that in the inherited form had been the head of a foot, namely the vowel of the final syllable of proparoxytone words. All the constraints favouring syncope were subject to prosodic constraints governing the distribution into coda and/or onset of a sequence of consonants that might result from the elimination of an internal post-tonic vowel; these constraints are the sonority sequencing constraint SONSEQ, the Syllable Contact Law (SYLCON), and the Minimum Sonority Distance constraint. Dominated by HEAD FOOT-RIGHT, the faithfulness and syllable structure constraints could not require the retention of two post-tonic central vowels. In certain contexts where Proto-Catalan had had two post-tonic central vowels, the result -- apocope or syncope -- is not completely general. While several interpretations accounting for 'irregularity' are proposed, there is insufficient data (absence of texts of the period of most interest, and lack of suitable relevant lexical examples to establish precise phonological conditions) to allow all of the details to be satisfactorily resolved.