C/C# - How to draw a triangle using the rubber band method?

Question

C++ / C#

Construct a triangle with given vertices. The points of the screen, which are the vertices of the triangle, are indicated from the keyboard using the rubber thread method.
What is the rubber thread method? I found two sources of similar programs - in one on Turbo Pascal it was necessary to control a point on the screen, press ENTER, place the second, third, and then they were connected by lines.
In the C# source, you could enter the coordinates into the fields on the screen, click on the button, and the triangle would appear.
Help understand.

Answer 1

There is even a guest for this: tdocs.su/23030
When constructing these figures, any of the vertices can be marked as fixed and, accordingly, any other vertex allows you to draw a triangle