Numbers help define NASCAR. Lap times and speeds are measured in miles per hour and seconds, and then there's historic numbers -- such as seven championships for Dale Earnhardt and Richard Petty, or 92 career wins (and counting) for Jeff Gordon.
Perhaps no numbers are more meaningful to fans, though, as car numbers. It's how fans spot their favorite drivers on the track, and it instills a sense of loyalty. It's why you see so many Earnhardt fans still holding on to all those "3" shirts.
Those car numbers don't always last an entire career, however.
For example, before the 2013 season, Matt Kenseth had driven the No. 17 for Roush Fenway Racing in every race from 2000-2012 (466 total). When he moved to Joe Gibbs Racing, he did not take the No. 17 with him. Instead, he got in a car with a number already allotted to JGR -- the 20. From 2008-2012, both Tony Stewart and Joey Logano had driven that car.
The numbers aren't owned by teams, though. NASCAR owns all numbers and assigns them to car owners. The sanctioning body maintains the right to revoke or transfer any number at any time.
Because NASCAR owns the rights to all numbers, car owners cannot assign a number they do not have or transfer a number.
Teams can request specific numbers, and NASCAR works with teams and sponsors to try and meet those requests, when able. For example, Richard Petty Motorsports has the rights to the No. 43, which team owner Richard Petty made famous during his legendary NASCAR career.
The numbers NASCAR doles out are good for one year via an owners license; generally, teams are re-issued the same numbers from year to year, if requested. If multiple teams request the same number, the team submitting the first entry form would generally have their request granted, although full-time racing status may also be considered.