The major studios, at first resistant to the double feature, soon adapted. All established B units to provide films for the expanding second-feature market. Block booking became standard practice: to get access to a studio's attractive A pictures, many theaters were obliged to rent the company's entire output for a season. With the B films rented at a flat fee (rather than the box office percentage basis of A films), rates could be set virtually guaranteeing the profitability of every B movie. The parallel practice of blind bidding largely freed the majors from worrying about their Bs' quality—even when booking in less than seasonal blocks, exhibitors had to buy most pictures sight unseen. The five largest studios—Metro-Goldwyn-Mayer, Paramount Pictures, Fox Film Corporation (20th Century Fox as of 1935), Warner Bros., and RKO Radio Pictures (descendant of FBO)—also belonged to companies with sizable theater chains, further securing the bottom line.