The top of the strike zone is defined as a horizontal line at the midpoint between the top of the batter's shoulders and the top of the uniform pants. The bottom of the strike zone is a line at the hollow beneath the kneecap. The right and left boundaries of the strike zone correspond to the edges of home plate. A pitch that touches the outer boundary of the zone is as much a strike as a pitch that is thrown right down the center. A pitch at which the batter does not swing and which does not pass through the strike zone is called a ball. A batter who accumulates three strikes in a single batting appearance has struck out and is ruled out. a batter who accumulates four balls in a single appearance has drawn a base on balls (a walk) and is awarded advancement to first base.