Each driver chooses an unassigned number from 2 to 99 (excluding 17 which was retired following the death of Jules Bianchi) upon entering Formula One, and keeps that number during his time in the series. The number one is reserved for the reigning Drivers' Champion, who retains his previous number and may choose to (but doesn't have to) use it instead of the number one. At the onset of the championship, numbers were allocated by race organisers on an ad hoc basis from race to race, and competitors did not have a permanent number throughout the season. Permanent numbers were introduced in to take effect in , when teams were allocated numbers in ascending order based on the Constructors' Championship standings at the end of the 1973 season. The teams would hold those numbers from season to season with the exception of the team with the World Drivers' Champion, which would swap its numbers with the one and two of the previous champion's team. New entrants were allocated spare numbers, with the exception of the number 13 which had been unused since . As teams kept their numbers for long periods of time, car numbers became associated with a team, such as Ferrari's 27 and 28. A different system was used from to : at the start of each season, the current Drivers' Champion was designated number one, his teammate number two, and the rest of the teams assigned ascending numbers according to previous season's Constructors' Championship order.