An application performs hit testing on regions to determine the coordinates of the current cursor position. Then it passes these coordinates — as well as a handle identifying the region — to the PtInRegion function. The cursor coordinates can be retrieved by processing the various mouse messages, such as WM_LBUTTONDOWN, WM_LBUTTONUP, WM_RBUTTONDOWN, and WM_RBUTTONUP. The return value for PtInRegion indicates whether the cursor position is within the given region.