One thing is universally common among bike racers regardless of the discipline or category: If you race, you’re going to be wearing a number. Racers wear numbers for identification purposes: who are you, are you registered for this event/category, etc. Rule 1N7 governs racing numbers. It requires promoters to make numbers available and defines the minimum size of the number and boarder space around the number. Rule 1N7 makes it the rider’s responsibility to place the number as prescribed by officials in such a way that they are visible when the rider is in the racing position.
Normally in a criterium or time trial that position is covering the side panel of your jersey on the side of the officials judging at the finish line. The most common errors in placement:
1) too high on back
2) upside down
3) wrong side of torso.
The most common excuse given for poor number placement: “my wife/girlfriend pinned it on me.” Although there has been a lot of tough talk about riders not being placed if they have not placed their number correctly, the truth is that the penalty for violation of rule 1N7 is not disqualification. However if you crumple the number or trim its edges (things riders do because it looks more “pro” or because they think it makes them more aerodynamic, but make the number difficult to read/film), you could pay a fine or at least the replacement cost for the number if noted before the start, or be relegated at the end of the race.