The method of making the center line with compasses is as follows:

1, draw a line segment with a ruler first.

2. Adjust the fixed length of the compass to a fixed value, which is more than half of the length of the line segment seen by naked eyes.

3. Draw two intersections on both sides of the line segment with compasses. For example, if the line segment AB is horizontal, first draw an arc above and below with A as the endpoint, and then draw an arc above and below with B as the endpoint, which has two focuses with the previous two arcs.

4. The line obtained by connecting two intersection points with a ruler is the midline of the line segment, also called the median vertical line, and the intersection point of the midline and the line segment is the midpoint of the line segment.