There is significant phonological variation among the various Yiddish dialects. The description that follows is of a modern Standard Yiddish that was devised during the early 20th century and is frequently encountered in pedagogical contexts. Consonants are bilabial, whereas are labiodental. The contrast has collapsed in some speakers. The palatalized coronals appear only in Slavic loanwords. The phonemic status of these palatalised consonants, as well as any other affricates, is unclear. and are velar, whereas are palatal. is an allophone of after , and it can only be syllabic . is an allophone of before . The phonetic realization of and is unclear: In the case of , puts it in the "velar" column, but consistently uses a symbol denoting a voiceless uvular fricative to transcribe it. It is thus safe to assume that is phonetically uvular . In the case of , puts it in the "palatalized" column. This can mean that it is either palatalized alveolar or alveolo-palatal . may actually also be alveolo-palatal , rather than just palatal. The rhotic can be either alveolar or uvular, either a trill or, more commonly, a flap/tap . The glottal stop appears only as an intervocalic separator. As in the Slavic languages with which Yiddish was long in contact (Russian, Belarusian, Polish, and Ukrainian), but unlike German, voiceless stops have little to no aspiration; unlike many such languages, voiced stops are not devoiced in final position. Moreover, Yiddish has regressive voicing assimilation, so that, for example, ('says') is pronounced and ('foreword') is pronounced . Vowels The vowel phonemes of Standard Yiddish are: are typically near-close respectively, but the height of may vary freely between a higher and lower allophone. appears only in unstressed syllables. The last two diphthongs may be realized as and , respectively. In addition, the sonorants and can function as syllable nuclei: אײזל 'donkey' אָװנט 'evening' and appear as syllable nuclei as well, but only as allophones of , after bilabial consonants and dorsal consonants, respectively. The syllabic sonorants are always unstressed. Dialectal variation Stressed vowels in the Yiddish dialects may be understood by considering their common origins in the Proto-Yiddish sound system. Yiddish linguistic scholarship uses a system developed by Max Weinreich in 1960 to indicate the descendent diaphonemes of the Proto-Yiddish stressed vowels. Each Proto-Yiddish vowel is given a unique two-digit identifier, and its reflexes use it as a subscript, for example Southeastern o11 is