I need to create a series of triangular ceiling planes.  What I can't figure out is how to make one from the rectangular plane, or to create a triangular plane directly.  I can use break line on the edges, but can't delete a point.  I tried using edit object parts to delete a segment and then connect the remaining edges but it does not work on ceiling planes.  Am I missing something very basic like the ability to delete a vertex of a polyline object?