It is precisely as catmcclure has deducted. So each block has one blade of a pinwheel at the top left, top right, bottom left and bottom right. If you want your large rectangle fabrics to be well distributed in your quilt and if you want the blades of each pinwheel to match (as in the pattern photo) you must set it all out on a design wall and are then dealing with a really fussy assembly process to make sure you sew each piece in the proper place (and the blades going the proper directions) to acheive this.