Athletes' bibs are used to identify them. I checked the IAAF rulebook for clarification on the particular rules of athletic bibs. There are five distinct rules, which are outlined below: There are five distinct rules, which are outlined below: Every athlete will be given two bibs, which must be worn conspicuously on the breast and back throughout competition, except in the jumping events, when only one bib may be worn on the breast or back. Bib numbers let race administrators distinguish between thousands of runners and ease the seemingly impossible chore of collecting a time for each and every one of them. Bib numbers are made of Tyvek (r), a durable, plastic-like paper product, and are meant to be fastened onto a t-shirt or racing vest while moving with the participants' bodies. Athletes' bibs are used to identify them. I checked the IAAF rulebook for clarification on the particular rules of athletic bibs.