線

1. Origins, structure, and basic meaning

• Structure: 線 (せん) can mean a drawn line or a transport route.
• Basic usage: Two points are connected with a straight line; a route runs along the coast.

2. Common pitfalls and similar expressions

• Geometric line and transport line
  • 線で結ぶ is visual means; この線は走る is a transport network. The latter is not ink on paper.