定規

1. Origins, structure, and basic meaning

• Structure: 定規（じょうぎ） means a ruler used to draw straight lines or measure length.
• Basic usage: Use it for a drawing tool or an item kept in a pencil case.

2. Common pitfalls and similar expressions

• Ruler versus tape measure
  • 定規 provides a straight edge for drawing lines; 巻尺 measures length with a flexible strip that can be rolled up.