Ian, Create a Plane through the two lines. Now Create a Curve on that Plane. Sketch an arc with the center at the lines' vertex and going from one line to the next. Now create a Datum Point at .500 Ratio on that Curve. Finally create your desired Plane through this new Datum Point, the lines' vertex and Normal to the Plane just created for sketching the arc. There are numerous other ways to construct this geometry; this is just one. If the lines are the same length, for example, you can sketch a line from end to end and create a Point at its midpoint. David