Baseball is a sport played between two teams usually of nine players each. It is a bat-and-ball game in which a pitcher throws (pitches) a hard, fist-sized, leather-covered ball towards a batter on the opposing team. The batter attempts to hit the baseball with a tapered cylindrical bat, made of wood (as required in professional baseball) or a variety of other materials (as allowed in many nonprofessional games). A team scores runs only when batting, by advancing its players—primarily via hits—counterclockwise past a series of four markers called bases arranged at the corners of a ninety-foot square, or "diamond." The game, played without time restriction, is structured around nine segments called innings. In each inning, both teams are given the opportunity to bat and score runs; a team's half-inning ends when three outs are recorded against that team.