We have:

bo, po, mo, fo, wo

But then:

duo, suo, luo, zuo, ruo, cuo, guo, huo, nuo, tuo

In each, the vowel is pronounced the same, but the creators of pinyin have chosen two different spellings. It seems that the u could be dropped from the second group without introducing ambiguity. So far, none of my Chinese friends have been able to convincingly explain this apparent inconsistency. Why does it exist?


After some discussion, both here and with native speakers IRL, it seems that I might have been incorrect in my assumption that both groups' vowel sounds are pronounced exactly the same. See jogoloran's and j5shi's answers that suggest a very subtle difference that occurs naturally due to initial position of the lips.

4 Answers 4


All the consonants in the first group are bilabials (articulated with the two lips).

The reason can't be phonemic, since there are no such Pinyin syllables *do, *so, *lo etc. -- as you correctly note Pinyin could be simplified by replacing all -uo syllables with -o.

I expect the reason for the spelling is perceptual -- from the perspective of phonetics, since the semi-vowel (the [w]) is a labial approximant, the glide is less noticeable when the consonant involved is bilabial to begin with.

  • Interesting! This is the first logically consistent grouping I've heard. Commented Mar 19, 2014 at 11:47
  • " as you correctly note Pinyin could be simplified by replacing all -uo syllables with -o." --- I don't agree, if they are the same, can I say I can simply replace all -o syllables with -ou?
    – j5shi
    Commented Mar 24, 2014 at 0:51
  • My point is that because there are no such syllables *do, *so, *lo, the syllables duo, suo, luo etc. could be written as such without introducing ambiguity.
    – jogloran
    Commented Mar 24, 2014 at 3:16
  • @jogloran you are right.
    – j5shi
    Commented Mar 24, 2014 at 8:43

It seems that Pinyin does this to conform to a former phonetic transcription system 注音符号, which drops u (ㄨ) before o (ㄛ) after b p m f.

注音符号 was originally designed to reflect some old (perhaps not even real) phonetic rules and symbols like uo (ㄨㄛ) uan (ㄨㄢ) were used. However, in most Chinese dialects there's no contrast between rounded and unrounded vowels after b p m f. So they decided to remove all u (ㄨ) and yu (ㄩ) after b p m f to simplify this system. [1] In fact (u)o is the only rounded vowel that can appear after b p m f. [2]

Another note: o (ㄛ) and e (ㄜ) are traditionally considered to be a rounded-unrounded pair, that's why 注音符号 deliberately chose similar symbols to represent them. In the Pinyin system, you will find “we” “o” are sometimes used interchangeably. e.g. weng = ong.

[1] 聲韻學:為何ㄅ、ㄆ、ㄇ、ㄈ不能與ㄨ的結合韻相拼?



[2] 唇音字的开合




what you asked is called compound finals, see here

And your case (o -> uo) is just one of the several cases, for example, a -> ia, e -> ie, o -> uo, a -> ua.

It has a slight difference in pronunciation, with compound finals, it sounds more smooth.

  • 1
    So are you saying that the vowel sound in bo is different from the vowel in suo? Commented Mar 20, 2014 at 1:50
  • 1
    Yes, they are slightly different, you shouldn't ignore 'u' in 'uo', 'uo' should be pronounced as "wo" while 'o' is just 'o' in English.
    – j5shi
    Commented Mar 20, 2014 at 2:36
  • @MikeChamberlain A little different. And because of the influence from the consonant (which determine the start position of tongue), "so" can't be directly pronounced without the transition of "u", so it should be "suo".
    – user4072
    Commented Mar 20, 2014 at 2:40
  • I thought they had a different sound too, but I didn't feel confident after reading the first comments and my difficulties outlined in my answer. I also no longer have any Chinese speakers nearby to consult. Commented Mar 21, 2014 at 12:11
  • 2
    The “o” after b/p/m/f does contain a glide /w/. The sound of “o” is the same as “uo” in Pinyin, except those in some interjections, such as “lo”, “yo” etc. which are often transcribed as “ao”, “ou” or “uo” instead. Pronouncing “bo” as /bo/ instead of /bwo/ will cause misunderstandings.
    – Yang Muye
    Commented Mar 21, 2014 at 14:46

I also struggle with all of these kinds of strange inconsistencies in Pinyin.

Your Chinese friends have trouble explaining it because they learned speaking before they learned Pinyin and Pinyin seems to have been designed to make sense that way at the cost of confusing adult second language learners.

With many languages such as English the writing is based on the idea of having certain letters for certain sounds, wherever they occur in a word. And because Pinyin uses our alphabet we expect it to work that way.

But in Chinese there's a different tradition based on syllables. Each syllable has a beginning sound and a final sound, also a medial sound. Some of these are optional so don't occur in every syllable.

Now there is actually greater variety in the sounds of these initials and finals, the sounds written with the same letters are often not quite the same, but they didn't want to have to use too many letters so decisions were made about which sounds would use the same letter and which would use different letters. There as more than one way to decide these things (linguists analyse the Chinese sounds typically three different ways), but Pinyin chose one of them.

For a native speaker all these sounds and combinations are already known and they just have to learn which letters to use for them.

But for a nonnative who is already used to the Latin alphabet from their native language we expect certain letters to each have one sound all the time, but in Chinese they actually don't.

This also explains a lot of bad foreigner pronunciations from people who learned Chinese with too much reading in pinyin and too little speaking with natives.

The best way to deal with it is to learn all the initial sounds, all the final sounds, and all the syllables made from them. But there a lot of them to learn! Way more than just learning the Pinyin letters, so it takes time and you first have to free your mind from your expectation of alphabetic spelling.

(And of course English spelling is nothing like one letter for one sound either. I suppose it might be harder for people who know a language with much more phonetic spelling to learn Chinese Pinyin.)

  • "With many languages such as English the writing is based on the idea of having certain letters for certain sounds, wherever they occur in a word." — English infamously does not fit this description, with spelling for different words based on different dialects at different points in time. However e.g. Spanish has a very strict phonetic orthography with one letter representing a single sound (except the consistent digraphs ch and ll like pinyin ch/sh/zh)
    – gnucchi
    Commented Jul 8, 2019 at 1:10
  • It's based on this idea. It's just worse at it than just about any other language. In any case, approaching Pinyin as though it's similar to Spanish orthography if you don't already have a solid grasp of the sounds of Mandarin, or without a good teacher, will lead to similar bad pronunciation as approaching it as though it's similar to English orthography. Commented Jul 8, 2019 at 3:10

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