Plumb

Plumb is a term used in construction and measurement to denote true vertical alignment with respect to gravity. As an adjective, it describes something perfectly vertical. As a verb, to plumb means to align, or to measure depth or verticality using a plumb line. The adverbial use "plumb" meaning completely or exactly is common in colloquial English.

The word derives from Old French plomb and Latin plumbum for lead, because early plumb lines used a lead weight. The connection remains in the term "plumber," historically a tradesman who worked with lead pipes.

A plumb line comprises a string or cord with a weighted bob; gravity pulls the bob to the vertical. When hung from a fixed point, the line indicates a true vertical, allowing alignment of walls, posts, or other features. Modern variants include laser or optical plumbs and digital levels, but the principle remains gravity-based.

In everyday language, "plumb" can extend to meaning "entire" or "completely," as in phrases like "plumb wrong" or "plumb impossible." The term also appears in architectural and surveying contexts to ensure verticality and proper drainage, plumbing, and structural integrity.