Modern Standard Chinese syllables are composed of an initial (聲母) and a final (韻母). A tone can be realized on the final as a pitch contour.
One of these finals is rather peculiar, i.e. the rhotic “er”, or [ɚ] in IPA. This final can only be combined with a zero-initial, i.e. no initial consonant. Therefore it is always just “er” by itself. Note that there are rhotic syllables (see below) where the “er” vowel is preceded by a consonant initial, e.g. shùgēr 樹根兒 (“root of a tree”), but these syllables are not basic morphemic level syllables since they are results of morphological processes called rhoticization. In fact, such syllables are still represented with two separate characters, just as shown in the example above. Therefore on a purely morphemic level, the “er” vowel cannot be preceded by a consonant initial.
In terms of its tones, the “er” can only have the 2nd, 3rd, and 4th tones, but not the 1st tone. Thus there are words like ér, ěr and èr, but no words like ēr. Here again, note that we are talking about basic morphemic level units, because as shown above in the example of shùgēr, it is possible to get a 1st tone syllable with the “er” vowel, but in these cases, the 1st belongs to the preceding syllable, i .e. “gēn” here, before the morphological fusion and phonological reduction of the suffix “ér”.
Now let’s call this final “er” the rhotic vowel Read More