Every team in Formula One must run two cars in every session in a Grand Prix weekend, and every team may use up to four drivers in a season. A team may also run two additional drivers in Free Practice sessions, which are often used to test potential new drivers for a career as a Formula One driver or gain experienced drivers to evaluate the car. Most modern drivers are contracted for at least the duration of a season, with driver changes taking place in-between seasons, in comparison to early years where drivers often competed at an ad hoc basis from race to race. Each competitor must be in the possession of a FIA Super Licence to compete in a Grand Prix, which is issued to drivers who have met the criteria of success in junior motorsport categories and having achieved of running in a Formula One car. Drivers may also be issued a Super Licence by the World Motor Sport Council if they fail to meet the criteria. Although most drivers earn their seat on ability, commercial considerations also come into play with teams having to satisfy sponsors and financial demands.