A:Answer S/M: Fits wrists measuring 130–180mm (about 5.1–7.1 inches). This shorter range suits smaller to average wrists, providing a snugger overall fit.
M/L: Fits wrists measuring 150–200mm (about 5.9–7.9 inches). This offers extra length for medium to larger wrists, with more adjustment holes for comfort.
The overlap (150–180mm) allows flexibility—opt for S/M for a tighter feel or M/L for looser. Apple recommends measuring your wrist with their printable tool for precision. Both sizes attach to the 42mm case identically, maintaining compatibility across Series 10 bands
