phonologists, there are some exceptions. For instance, and are found in closed syllables ending in , and only is found in closed monosyllables before , , and . The Parisian realization of has been variously described as central and centralized to before , in both cases becoming similar to . The phonemic opposition of and has been lost in the southern half of France, where these two sounds are found only in complementary distribution. The phonemic oppositions of and and of and in terminal open syllables have been lost in almost all of France, but not in Belgium or in areas with an Arpitan substrate, where and are still opposed as and . Open vowels The phonemic contrast between front and back is sometimes not maintained in Standard French, which leads some researchers to reject the idea of two distinct phonemes. However, the distinction is still clearly maintained in other dialects such as Quebec French. While there is much variation among speakers in France, a number of general tendencies can be observed. First of all, the distinction is most often preserved in word-final stressed syllables such as in these minimal pairs: → ('stain'), vs. → ('task') → ('leg'), vs. → ('paste, pastry') → ('rat'), vs. → ('short') There are certain environments that prefer one open vowel over the other. For example, is preferred after and before : ('three'), ('gas'). The difference in quality is often reinforced by a difference in length (but the difference is contrastive in final closed syllables). The exact distribution of the two vowels varies greatly from speaker to speaker. Back is much rarer in unstressed syllables, but it can be encountered in some common words: ('castle'), ('past'). Morphologically complex words derived from words containing stressed do not retain it: → ('aged', from → ) rarissime → ('very rare', from → ). Even in the final syllable of a word, back may become if the word in question loses its stress within the extended phonological context: J'ai été au bois → ('I went to the woods'), J'ai été au bois de Vincennes → ('I went to the Vincennes woods'). Nasal vowels The phonetic qualities of the back nasal vowels differ from those of the corresponding oral vowels. The contrasting factor that distinguishes and is the extra lip rounding of the latter according to some linguists, and tongue height according to others. Speakers who produce both and distinguish