Jack Nardi shared this technique to create a radius between parallel lines in AutoCAD.
“I often need to generate 2D linkages, and I prefer a radius at each end. If you draw a line and offset it a specific distance (mine are usually 1"), then use the zero fillet to pick the adjacent end points, AutoCAD will add a radius to the end so you can generate a closed linkage quickly. This function has been part of AutoCAD for many versions and was originally listed in their reference manual.”
Notes from Cadalyst tip reviewer Brian Benton: The AutoCAD fillet tool is very useful. This tip is another way it can be used. In this case, AutoCAD creates a fillet between the two selected lines. They are parallel and the ends line up. The Fillet command will create the arc at whatever radius is needed, with no thought required from the operator, and draw it.