The word 口径 is formed by combining two characters, each contributing a core physical concept to create a new, integrated idea. 口 (kǒu) means "mouth" or "opening", directly indicating an orifice or the end of a tubular object. 径 (jìng) means "diameter" or "path", conveying a sense of a straight-line measurement or a defined course. Together, they metaphorically extend from the literal "diameter of an opening" (like a gun barrel or pipe) to represent the standardized measurements, specifications, or the figurative "caliber" of ideas, statements, or policies, forming a unified concept of a defining gauge or standard.