What does the delta in Deltabox mean?

“Delta means the triangular shape of the frame formed by the three points of the lower and upper ends of the head pipe and the rear pivot point. And, because the cross-section of the frame is a box shape, it is called a Deltabox frame. The first frame with this basic shape was used on a YZR500 Yamaha GP500 (present MotoGP) class race machine back in 1983, and it proved to be a good frame for improving the machineʼs handling stability. Since then the shape has evolved in various ways and it has become the basic frame type used on most of the worldʼs supersport bikes.”