Tires called 20” can be one of two sizes. The 406 bead seat diameter was commonly used on bikes from France, Holland and the US. Typically BMX, juvenile, folding, and recumbent bicycles as well as some trailers need a 406. The 451 size was used on bikes from Italy and Britain as well as Schwinn S-5 and S-7 bicycles. In Italy it’s know as the 500x35A. In Britain E.5J, EA18.0, R18.0, and EB18.0. If you’re not sure which size you need please go to our helpdesk and send us a question.