law, rule, door post
臬 centers on a fixed standard or boundary marker: a law or rule that sets limits, and by extension a door post that marks an entrance. The meanings connect through the idea of a firm upright that defines a threshold.
The exact historical development of 臬 is uncertain. It may combine 自 (nose/self) and 木 (tree/wood) to suggest a wooden post used as a standard or marker, but this is speculative.
Picture a wooden post (木) with a nose-like shape (自) on top, standing as a fixed marker. This post represents the law or rule that defines the boundary, like a door post marking the entrance.
For ゲツ, imagine a guard standing by the door post, and the guard says 'Gets!' as a sharp command to stop. The sound 'Gets' cues ゲツ, and the guard enforces the rule at the post.
—