One of the features of the polyline object is that you can turn edges into arcs when you need a curve, and I was trying to make a "D" shape. Had to add a third vertex in the middle of the flat side before it would let me.
Now I think of it, I think I remember: it used two points, with arcs instead of line segments, and "closed" to get the second arc. I'm not sure if you could force the closing segment back to a straight one, but that could have probably got your "D" shape, if so.
Here's another odd note though, if you have a polyline with a single arc and use properties to set it to closed, it adds a straight edge and you get a D shape. If you use PEDIT CLOSE it adds another arc to close it instead, and it tries to continue tangent at the end, completing a whole circle.