Line Pen

Create > Polygons > Line Pen - Line Pen is a polygonal sketching tool. If you use it to create interconnected lines, it will create a two-point poly chain, useful for hair guides and dynamics. If you connect lines to make closed shapes, Line Pen will create polygons. If you have existing polygons and draw additional lines attached to them, Line Pen will make those lines into polygons too.

Lines do not have to be sketched in a void. You can draw on existing polygons and holding Shift as you draw allows you to draw out from the normal of the polygon you are drawing on. Holding Ctrl will constrain the lines to an axis, in common with other Modeler tools.