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A watch strap taper refers to the gradual reduction in width from the lug end to the buckle end of the strap, not all watch straps have a taper.
For example, if a watch strap has a lug width of 20mm and a taper of 2mm, it means that the strap's width gradually reduces by 2mm at the buckle end, resulting in a final width of 18mm at the buckle, so would be refered to as 20mm/18mm.