What do the tapered sizes mean?
Tapered straps start out wider at the lugs (the end that attaches to your watch) and narrow (taper) toward the direction of the tail and the buckle. So for example, if your Rolex Sumbariner has a 20mm lug width and you want a tapered strap, then you would choose a 20/18 strap. This means the lug is 20mm wide and the buckle width is 18mm. Keep in mind that the higher number is the lug width and the smaller number is the buckle/tail width.