The shape of a Line segments cross-section is described by a series of line segments.

Sample 1
One half of the shape of the cross-section is initially based on line segments arranged in a trapezoid shape. Points 1 and 4 are the fix start- and endpoint.
All corner points are connected by line segments. The coordinates of each point and the related corner radius can be adjusted in the context dialog:

Coordinates and radius of vertex
Using the context menu of a line segment, points can be added at the cursor position or be removed:
1.
2. 
The context menu also offers to display and edit the values of the segment. Either the start- or endpoint of the segment can be changed. In some cases, like in sample 1, the segment between point 1 and 2 has a fixed start point according to the geometrical constraints.

When moving points the following constraints can be enforced by pressing a key on keyboard:
CTRL |
Point moves on a circle around the previous point. The radius stays constant while pressed. |
CTRL + SHIFT |
Point moves on a circle around the next point. The radius stays constant while pressed. |
ALT |
Point moves on a line between its last position and previous point. |
ALT + SHIFT |
Point moves on a line between its last position and next point. |
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