Locations of window rectangles are described by RECTL structures, which contain the coordinates of two points that define the lower-left and upper-right corners of the rectangle. An empty rectangle is one that has no area: either its right coordinate is less than or equal to its left coordinate, or its top coordinate is less than or equal to its bottom coordinate.

There are two types of rectangles in OS/2: inclusive-inclusive and inclusive-exclusive. In inclusive-exclusive rectangles, the lower-left coordinate of the rectangle is included within the rectangle area, while the upper-right coordinate is excluded from the rectangle area. In an inclusive-inclusive rectangle, both the lower-left and upper-right coordinates are included in the rectangle.

In general, graphics operations involving device coordinates (such as regions, bit maps and bit blts, and window management) use inclusive-exclusive rectangles. All other graphics operations, such as GPI functions that define paths, use inclusive-inclusive rectangles.