Word-initial consonants are geminated after certain vowel-final words in the same prosodic unit. There are two types of triggers of initial gemination: some unstressed particles, prepositions, and other monosyllabic words, and any oxytonic polysyllabic word. As an example of the first type, "casa" ('house') is pronounced but "a casa" ('homeward') is pronounced . This is not a purely phonological process, as no gemination is cued by the "la" in "la casa" 'the house' , and there is nothing detectable in the structure of the preposition "a" to account for the gemination. This type normally originates in language history: modern "a", for example, derives from Latin AD, and today's geminate in is a continuation of what was once a simple assimilation. Gemination cued by final stressed vowels, however, is transparently phonological. Final stressed vowels are short by nature, if a consonant follows a short stressed vowel the syllable must be closed, thus the consonant following the final stressed vowel is drawn to lengthen: "parlò portoghese" 's/he spoke Portuguese' vs. "parla portoghese" 's/he speaks Portuguese'.