One of the characteristics of ancient Greek geometry is the method of drawing straightedge and straightedge, which is transformed into modern algebra (I won't go into details here, you can learn more about it yourself and read related books if you are interested). In fact, ruler and ruler drawing can be regarded as two operation modes:

1. Rulers are used to draw or extend straight lines, and cannot be used for other purposes, including unmeasurable lengths;

The compass is used to draw a circle, but it can't measure the angle. However, because the compasses can be fixed after the feet are opened, they can be used to intercept a certain length, which ensures that the radius of the circle will not change when drawing a circle, and also provides a means of drawing-that is, intercepting the length of the line segment with compasses.

As can be seen from the above two regulations, if you want to intercept a fixed length on a straight line or line segment, you can only use the compass method.