Straight line: In daily life, a tight rope, a bamboo pole and a crosswalk all give us the image of a line segment. In fact, a straight line extends indefinitely and its length cannot be measured.

trait

◆ There are no endpoints at both ends.

◆ A straight line can extend infinitely to both ends.

◆ The length of a straight line cannot be measured.

In the academic sense, a line is the locus of a point and a surface is the locus of a line.

When you say that a straight line is thick and 5 or 6 centimeters wide, it is no longer an academic straight line, it is materialized.

Since it is materialized, it becomes an object. An object cannot be called a straight line or a plane, but can only be described by a straight line and a plane.