An ideal line, the shadow-line: it is immaterial & unexpansive.

Different types of lines can be constructed with algebra. They can resemble wave-forms, zig-zag forms and others.
Here they serve the purpose to demonstrate that the "free line" is the mother of all lines. The "free line" (Freie Line) is flexible and can become all other lines, a curve, zigzag etc. A curve will allways be a curve.