When you sing Peking opera you must keep to the beat.
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culture
In opera music 板 is the strong beat, struck on the clapper, and 眼 the weak beats: 一板三眼 is four beats to the bar, 一板一眼 two. Keeping to the beat came to mean doing things properly, which gave the idioms 一板一眼 (by the book) and 有板有眼 (well ordered).
