plummet; plumb line
A weight suspended from a string used to establish a vertical reference line. This is a technical term in carpentry, surveying, and construction. The kanji 垂 (hang down) and 準 (standard) directly convey the meaning.
The carpenter used a plumb line to check if the wall was vertical.
Composed of 垂 (hang down) and 準 (standard, level). The reading すいじゅん is the on'yomi of both characters. The word is a technical compound, likely coined in the context of traditional Japanese carpentry or surveying.