distinguished regionally, chiefly in Belgium). A fourth vowel length system, still non-phonemic, has now arisen: All nasal vowels as well as the oral vowels (which mostly derive from former long vowels) are pronounced long in all stressed closed syllables, and all vowels are pronounced long in syllables closed by the voiced fricatives . This system in turn has been phonemicized in some varieties (e.g. Haitian Creole), as a result of the loss of final . Latin diphthongs The Latin diphthongs ae and oe, pronounced and in earlier Latin, were early on monophthongized. ae became by the 1st century at the latest. Although this sound was still distinct from all existing vowels, the neutralization of Latin vowel length eventually caused its merger with < short e: e.g. caelum "sky" > French ciel, Spanish/Italian cielo, Portuguese céu , with the same vowel as in mele "honey" > French/Spanish miel, Italian miele, Portuguese mel . Some words show an early merger of ae with , as in praeda "booty" > *prēda > French proie (vs. expected **priée), Italian preda (not **prieda) "prey"; or faenum "hay" > *fēnum > Spanish heno, French foin (but Italian fieno /fjɛno/). oe generally merged with : poenam "punishment" > Romance * > Spanish/Italian pena, French peine; foedus "ugly" > Romance * > Spanish feo, Portuguese feio. There are relatively few such outcomes, since oe was rare in Classical Latin (most original instances had become Classical ū, as in Old Latin oinos "one" > Classical ūnus) and so oe was mostly limited to Greek loanwords, which were typically learned (high-register) terms. au merged with ō in the popular speech of Rome already by the 1st century . A number of authors remarked on this explicitly, e.g. Cicero's taunt that the populist politician Publius Clodius Pulcher had changed his name from Claudius to ingratiate himself with the masses. This change never penetrated far from Rome, however, and the pronunciation /au/ was maintained for centuries in the vast majority of Latin-speaking areas, although it eventually developed into some variety of o in many languages. For example, Italian and French have as the usual reflex, but this post-dates diphthongization of and the French-specific palatalization > (hence causa > French chose, Italian cosa not **cuosa). Spanish has , but Portuguese spelling maintains , which has developed to (and still remains as in some dialects, and in others). Occitan, Romanian, southern Italian languages, and