carpenter's square, ruler, rule
榘 centers on a tool for measuring and guiding: a carpenter's square, a ruler, or a standard rule. The meanings all point to an instrument that ensures straightness and correctness.
榘 is a variant form of 矩, which originally depicted a carpenter's square. The addition of 木 (wood) likely emphasizes the material of the tool, but the exact historical development is uncertain.
The top part 矩 looks like a measuring tool, and the bottom 木 is wood. Picture a wooden carpenter's square used to mark straight lines and set rules.
For ク, imagine a carpenter using a square to check a cool wooden frame: cool -> ク, and the square ensures everything is straight.