直径

1. Origins, structure, and basic meaning

• Structure: 直径 (ちょっけい) is a mathematical noun for the length through the center of a circle or cylinder from edge to edge.
• Basic usage: 皿は直径が二十センチ makes a plate’s dimension the stated value, while 幹の直径を測る makes trunk width through its center the object measured.

2. Common pitfalls and similar expressions

• Flat circle and trunk cross-section
  • 皿の直径 and 幹の直径 both cross a center despite different objects. Diameter differs from circumference and from radius, which ends at the edge.