Since 1984 Formula One teams have been required to build the chassis in which they compete, and consequently the terms "team" and "constructor" are more or less interchangeable. This requirement distinguishes the sport from series such as IRL, Champ Cars, and NASCAR, which allow teams to purchase chassis, and "spec series" such as GP2, which require all cars be kept to an identical specification. In its early years, Formula One teams sometimes also built their engines, though this became less common with the increased involvement of major car manufacturers such as BMW, Mercedes-Benz, Renault, Toyota, and Honda, whose large budgets rendered privately built engines less competitive (and redundant). Early manufacturer involvement came in the form of a "factory team" (that is, one owned and staffed by a major car company), such as those of Alfa Romeo, Ferrari (FIAT) or Renault. Companies such as Climax, Repco, Cosworth, Hart, Judd and Supertec, which had no direct team affiliation, often sold engines to teams who could not afford to manufacture them. As the manufacturers' deep pockets and engineering ability took over, almost all engines are now produced by major manufacturers. After having virtually disappeared by the early 1980s, factory teams made a comeback in the 1990s and 2000s, and now form half the grid with Toyota, Ferrari (FIAT), Honda, Renault and BMW either setting up their own teams or buying out existing ones. Mercedes-Benz (DaimlerChrysler) owns 40% of the McLaren team and manufactures the team's engines. Commercial engine supplier Cosworth exited the sport at the end of 2006. Thus all the teams will run on factory supplied engines from 2007. The sport's 1950 debut season saw eighteen teams compete, but due to high costs many dropped out quickly. In fact, such was the scarcity of competitive cars for much of the first decade of Formula One that Formula Two cars were admitted to fill the grids. Ferrari is the only still-active team which competed in 1950, and as of 2006 eleven teams remain on the grid, each fielding two cars. Although teams rarely disclose information about their budgets, it is estimated that they range from US$66 million to US$400 million each. Entering a new team in the Formula One World Championship requires a Ј25 million (about US$47 million) up-front payment to the FIA, which is then repaid to the team over the course of the season. As a consequence, constructors desiring to enter Formula One often prefer to buy an existing team: B.A.R.'s purchase of Tyrrell and Midland's purchase of Jordan allowed both of these teams to sidestep the large deposit. Each car is assigned a number. The previous season's World Drivers' Champion is designated number 1, with his teammate given number 2. Numbers are then assigned according to each team's position in the previous season's World Constructors' Championship. There have been exceptions to this rule, such as in 1993 and 1994, when the current World Drivers' Champion (Nigel Mansell and Alain Prost, respectively) was no longer competing in Formula One. In this case the drivers for the team of the previous year's champion are given numbers 0 (Damon Hill, on both occasions) and 2 (Prost himself and Ayrton Senna - replaced after his death by David Coulthard and occasionally Nigel Mansell - respectively). The number 13 has not been used since 1974, before which it was occasionally assigned at the discretion of individual race organizers. Before 1996, only the world championship winning driver and his team generally swapped numbers with the previous champion – the remainder held their numbers from prior years, as they had been originally set at the start of the 1974 season. For many years, for example, Ferrari held numbers 27 & 28, regardless of their finishing position in the world championship. As privateer teams quickly folded in the early 1990s, numbers were frequently shuffled around, until the current system was adopted in 1996. Michael Schumacher holds the record for having won the most Drivers' Championships (seven) and Ferrari holds the record for having won the most Constructors' Championships (fourteen). Jochen Rindt became the only posthumous World Champion after a fatal accident at the 1970 Italian Grand Prix.