Jersey numbers: why the digits run the way they do
Numbering
The link between hand signals at the scorer's table and the numbers allowed on a shirt, and what changed when the restriction was lifted.
For decades, international competition allowed squad numbers only from 4 to 15. The reason was not aesthetic. When an official calls a foul, he reports it to the scorer's table by signalling the number of the offending player with his fingers, and fingers only go up to five per hand.
The system works like this. A number below 10 is shown with one hand. A number of two digits is shown in two parts, tens first, then units, with a brief pause between them. Number 12 is one finger, then two. Number 15 is one finger, then a closed fist for five, or a spread hand depending on the convention. Numbers 1, 2 and 3 were banned precisely because officials already use one, two and three fingers to signal the number of free throws awarded, and confusion at the table costs everybody a stoppage.
The restriction was lifted in most competitions, and squads can now use 0, 00 and any number from 1 to 99, though not both 0 and 00 on the same roster. Signalling adapted: officials now report larger numbers in the same two part sequence, and the table screens are better than they were. The old range survives anyway, because players who grew up in the system keep the numbers they wore as juniors.
One rule that has not changed is uniqueness within a squad. No two players on the same roster may wear the same number in a competition, and the number on the front and the back of the shirt must match. Numbers must also be a solid colour contrasting with the shirt, which is why you rarely see outlined or patterned digits in serious competition.
There is a practical viewing benefit in knowing the signalling sequence. When a foul is called and you cannot tell who it was on, watch the official's hand at the table rather than the replay. Two fingers, pause, four fingers means twenty four. The player raises his hand at the same moment to acknowledge the call, which is a convention rather than a rule, but it is nearly universal and it tells you who is now one foul closer to sitting down.
Number size and placement are specified too, with a minimum height for the digits on the back and a smaller minimum on the front, so they stay legible from the top row of a hall. It is one of the few parts of a kit that is genuinely regulated rather than designed.