Jersey numbers: why 0 and 00 are different, and where 1 to 3 went
Numbering
The hand-signal logic behind traditional number ranges, and why rulebooks have been loosening it.
Referees report fouls and baskets to the scorer's table with their hands. Two raised fingers means number two, five fingers means five, a fist followed by three fingers means three. The whole system is built on digits, and that is where the old restrictions on jersey numbers came from.
Because a referee can only show digits zero through five cleanly on one hand, older rulebooks limited numbers to combinations of those digits: 0 through 5, then 10 through 15, 20 through 25, and so on up to 55. That is why you still see so many players in the 20s and 30s and almost none in the 60s or 70s in competitions that kept the convention. A number like 68 could not be signalled without two awkward gestures.
International rules went the other way for a while and reserved 1, 2 and 3 entirely, because those same signals mean one, two and three free throws awarded. Teams there once started numbering at 4. That restriction has since been relaxed in most competitions, and ranges up to 99 are now widely permitted, with referees using a two-hand sequence for the larger numbers.
Zero and double zero are treated as separate numbers, which trips people up. Two players on the same roster can wear 0 and 00 at the same time, and a referee distinguishes them by signalling a closed fist once or twice. It is a quirk preserved from the same hand-signal logic rather than a design decision.
Numbers also carry a size requirement that rarely gets mentioned. Digits on the back have to be a specified height, usually around six inches, with front numbers slightly smaller, and there is a minimum stroke width so a number stays legible from the far corner of an arena. That is why jersey typography tends toward heavy, open block forms with simple counters. A thin or heavily stylised numeral fails the moment somebody is moving.
If you are designing anything that imitates the look, the constraint to respect is legibility at distance and at speed, not decoration. The best number sets read instantly when the wearer is turned halfway away from you, which is most of the time.