a passed ball ('PB') on a strikeout, where the player is allowed to run and reach base safely if he can. When the batter hits a fair ball, he must run to first base and may continue or stop at any base unless he is put out. A successful hit occurs when the batter reaches a base: reaching only first base is a single; reaching second base, a double; third base, a triple; and a hit that allows the batter to touch all bases in order on the same play is a home run—whether the ball is hit over the fence does not matter (if the ball is not hit over the fence and the batter touches all bases, it is usually referred to as an "inside-the-park home run"). Once a runner is held to a base, he may attempt to advance at any time, but is not required to do so unless the batter or another runner displaces him (called a force play). A batter always drops his bat when running the bases; otherwise, the bat would slow him down and could give rise to a call of interference if it were to contact the ball or a fielder. However, if a batter hits the ball, and the batter or the dropped bat touches the ball, it is considered a dead ball. Depending on the way the ball comes off the bat, the play has different names. A batted ball is called a fly ball if it is hit in the air in an upward arc, such that a fielder might be able to catch it before it hits the ground. A batted ball is called a ground ball if it hits the ground within the infield before it can be caught, often due to being hit in a downward trajectory. Several different names are used to describe fly balls, depending on their trajectory. A ball hit high in the air and seemingly almost straight up is called a "pop-up". A ball hit forcefully in a fast-moving and seemingly almost straight-line trajectory is called a line drive. A "shallow" fly ball, hit with just enough force to possibly land between the infielders and the outfielders, is often called a "blooper". A "deep" fly ball is hit with enough force to approach and possibly clear the outfield fence. When a ball is hit outside the foul lines, it is a