ruler, carpenter's square
矩 centers on a tool for measuring right angles and straight lines: a ruler or carpenter's square. The idea extends naturally to the standard or rule itself.
矩 combines 矢 (arrow) and 巨 (large), but the historical origin is uncertain. The modern form may be a simplification or rebus, and the connection to a carpenter's square is not directly visible from the components.
An arrow 矢 is straight, and 巨 suggests something large or imposing. Picture a giant arrow used as a ruler to draw perfectly straight lines and right angles.
For ク, imagine a cool carpenter using a square ruler: cool -> ク, and the tool helps him measure precisely.
rectangle
quadrature
carpenter's square
following the desires of one's own heart without transgressing what is right
standard; rule
carpenter's square
square wave; rectangular wave
quadrature
standard; criterion; rule; norm
rules and standards; norms; criteria