What are the rules governing expressions such as 七八成 (seventy or eighty percent) or 一两个 (one or two)? Must the numerals be monosyllables? What are the restrictions on the measure words? Can there be at most two numerals?
Must the numerals be monosyllables?
In my opinion, you can use multi-sllable numbers, like:
十五六公里 (fifteen or sixteen kilometers)
百八十个 (one hundred or eighty)
三十七八岁 (thirty-seven or thirty-eight years old)
Restrictions on the measure words?
No, I have no idea.
Can there be at most two numerals?
Yes, I have never seen more than 2 numbers used in such a pattern. This pattern is used to express an estimation, an approximation.
You raise an interesting point with the second example: in the other two, the values only differ by one — in the middle example, they differ by twenty, and the first value is greater than the second. Is that another constraint? Could I say 十五七公里 to mean "fifteen or seventeen kilometres"? Could I say 八十百个 to mean "eighty or a hundred"? I find this intriguing, as I couldn't find anything in the literature about this pattern.– jogloranFeb 27, 2012 at 11:32
1@jogloran In general, the pattern is [numberA][numberA+1][measure word], so 十五七公里 is wrong, however, there are very few exceptions like 百八十. Such exceptions are fixed, so you can't say 八十百 or 百九十. Also, in classic Chinese poems, the order of the numbers may invert to meet the format of poems. I.e,竹外桃花三两枝.(by famous poet/writer/politician 苏轼)– HuangFeb 27, 2012 at 11:39
Fascinating. Another clarification: is 十九二十个 correct for 'nineteen or twenty', or must the first and second numbers only differ in the units place?– jogloranFeb 28, 2012 at 4:39
1@jogloran 十八九个 sounds good, while 十九二十个 sounds a little strange, but I think I will understand it in the similar sense of "十八九个". You maysay "二十个左右" or "大约二十" or even "十九或二十个" to express that meaning.– HuangFeb 28, 2012 at 13:40
I think English usage can serve as a good reference here. In English, saying "7 or 8" sounds normal as a way of expressing an estimate, but saying "7 or 9" does not sound normal as a way of expressing an estimate (instead you would express it as a range of 7 to 9).
Likewise, saying "7, 8, or 9" in English does not sound like an estimate but rather like a specific range or a specific set of choices.
I think the same holds true for Chinese.