What does it mean to draw the middle line in red?

This is a broken line-a special broken line that cuts off and hides some objects.

In industrial drawing, the workpiece is too big and too long to be displayed, and the part that does not need to be displayed is interrupted by a line on the drawing, which is called the cutoff line.

The cut-off line usually consists of two straight lines with a zigzag line in the middle. One side of the cutoff line has graphics, and the other side is hidden from display.

Cut-off line is often used in mechanical drawing and architectural drawing, and is one of the indispensable elements in drawing drawings.

When learning drawing, you must be familiar with and master the symbols of drawing, so that you can draw and read drawings. I hope it helps you.