Lettering

Each sticker that can be a target gets a letter. The common scheme is Speffz: A to D on the top face, E to H on the left, I to L on the front, M to P on the right, Q to T on the back and U to X on the bottom, going clockwise round each face from its top-left piece. Corners and edges use the same letters, so a letter names a sticker only together with the piece kind. The same rule letters the 4x4's and 5x5's wings and centres.