plumb line
noun [ C ] uk/ˈplʌm ˌlaɪn/ us/ˈplʌm ˌlaɪn/
（测垂直面或水深的）铅垂线，重锤线
a piece of string with a weight attached to one end, used either to test if something vertical, such as a wall, is exactly straight, or to find the depth of water
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