In GDI the right and bottom edges of a RECT are adjacent to the region being specified by the RECT. So the right most pixel in the region will be at RECT.X – 1.
DirectX also uses RECTs to designate rectangular regions. Does DirectX follow the same convention as GDI with respect to the exclusivity of the right and bottom edges? From some preliminary testing I’d say no – RECTs are inclusive along all edges in DirectX. I’ll need to do more testing to be sure of this.