Carpentry: water level

Self-tutoring about reliable, elegant tools: the tutor mentions the idea of a water level.

A water level is a long tube, almost totally filled with water, with clear spans of tubing at each end. The idea is that when one holds both ends pointing up, so no water is escaping from either end, then the water height at both ends will be the same.

Water levels can be used to find places of equivalent height on fence posts, walls around corners, etc.

Source:

YouTube: TedsDixieSheds: How to use a Water Level – Construction Tips from Ted