Topic Title

parallel lines using the first line picked as the control, with the resulting arc having a radius of one half the distance between the lines.

Arc/Tangent does this!

Arc/tangent is close or is the same if the line ends are the same. If not, then it is tangent only at one end.
Thanks though.
Woody

If you have Object Snap\Tangent toggled on, it will work if you start on the shorter line. To start with the longer line, you can use the Shift-Snap Get X or Get Y to finish, and then 2-Line Trim to connect the arc to the shorter line.