In pinyin, why is there an inconsistency between bo, po, suo, duo etc

Question

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?

EDIT

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.

Answer 1

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.

Answer 2

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]

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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？

民國10年（1921年）2月國語統一籌備會訂正、教育部公布之《教改國音字典》（商務印書館本）附〈修正《國音字典》之說明〉一文，云：

又如唇音各聲母與韻母拼合，原案根據韻書，有用開口呼之韻母者，有用合口呼之韻母者，如「剝」「駁」音「ㄅㄛ」，「撥」「鉢」音「ㄅㄨㄛ」；「煩」「繁」音「ㄈㄨㄢ」，「凡」「帆」音「ㄈㄢ」之類；編《字典》時，因唇音聲母本（巳）［已］合口，故遇原案中唇音與合口呼韻母拼合之字，除「ㄨ」韻以外一律改用開口呼韻母。

[2] 唇音字的开合

因此，唇音字没有开合口的对立。赵元任《中古汉语的语音区别》和李荣《切韵音系》中都主张中古汉语唇音字不分开合口。虽然原则上没有对立，但是在开合口分韵而且原来的主要元音是央、后元音的韵部中，唇音声母后有一个介音。

这和现代汉语普通话的情况非常类似。其他位置的声母都有有无介音[u]的对立，如“单”和“端”、“甘”和“关”、“三”和“酸”、“詹”和“砖”、“安”和“湾”，只有唇音原则上没有不分开合的对立。

Answer 3

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.

Answer 4

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.)