Learn something new every day: order of components in some Chinese two-character phrases

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I was reading the Wikipedia article on Standard Mandarin and came across this bit:

It is known that if the two morphemesof a compound word cannot be ordered by grammar, the order of the twois usually determined by tones — Yin Ping (1), Yang Ping (2), Shang(3), Qu (4), and Ru, which is the plosive-ending tone that has alreadydisappeared. Below are some compound words that show this rule. Tonesare shown in parentheses, and R indicates Ru.

左右 (34)
南北 (2R)
輕重 (14)
貧富 (24)
凹凸 (1R)
喜怒 (34)
哀樂 (1R)
生死 (13)
死活 (3R)
陰陽 (12)
明暗 (24)
毀譽 (34)
褒貶 (13)
離合 (2R)

Fascinating!