口径
koǔ jìng · HSK 7
diameter of opening
🔫 口 = opening/mouth + 径 = diameter/path; the diameter of an opening = caliber
口 — pictograph of an open mouth
口 is one of the oldest pictographs in Chinese, originally drawn as a simple outline of an open mouth. Its core meaning 'mouth' extended naturally to 'opening' or 'entrance', as in 入口 (entrance). It also serves as a common radical in characters related to speech, eating, or apertures.