 At one time F1 regulations regarding the changing of drivers during the course of a season were extremely liberal, which encouraged some teams to recruit a string of pay drivers to drive their cars, sometimes only for one or two races.Frank Williams Racing Cars (the predecessor to Frank Williams and Patrick Head's highly successful Williams F1 team) were particularly prolific with regard to the number of drivers they would use in a season - ten drivers drove for the team in both 1975 and 1976.Because of this the rules on driver changes were subsequently tightened.