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.
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
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.