Spanish stress shows a uniform pattern in verbal forms. For stressed roots in particular, the unmarked position is the last vowel. Nevertheless, first conjugation roots ending in a high vowel display two opposite behaviours: while one group follows the unmarked pattern (e.g. envío 'I send'), the other group keeps the high vowel unstressed, thus always becoming a glide (e.g. cambio 'I change'). While nominals related to the latter group exhibit the same stress position as the first conjugation (e.g. cambio [kámbjo] 'change', cambio [kámbjo] 'I change'), this is not true of nominals related to the former group, since we find some cases with stress shift (e.g. amplio [ámpljo] 'large', amplío [amplío] 'I enlarge') and other cases without this change (e.g. envío [embío] 'shipment', envío [embío] 'I send'). The goal of this paper is to account for these facts (including the lack of pairs such as *amplío [amplío] 'large' and *amplio [ámpljo] 'I enlarge') and analyze the prosodic and morphological factors determining glide formation.

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