There is a set of GL commands to support efficient specification of rectangles as two corner vertices.
void Rect[sifd] ( T x1, T y1, T x2, T y2 ) ;
void Rect[sifd]v ( T v1[2], T v2[2] ) ;
Each command takes either four arguments organized as two consecutive
pairs of 
coordinates,
or two pointers to arrays each of which contains an x value followed by
a y value.
The effect of the Rect command
is exactly the same as the following sequence of commands:
The appropriate Vertex2 command would be invoked depending on which of the Rect commands is issued.