A wide receiver is an offensive position in American and Canadian football, and is the key player in most of the passing plays. Wide receivers are among the fastest players on the field. Only players in the backfield or the ends on the line are eligible to catch a forward pass. The two players who begin play at the ends of the offensive line are eligible receivers, as are all players in the backfield. The backs and ends who are relatively near the sidelines are referred to as "wide" receivers. At the start of play, one wide receiver may begin play in the backfield, at least a yard behind the line of scrimmage. Such positioning allows another player, usually the tight end, to become the eligible receiver on that side of the line. Such positioning defines the strong side of the field.