A double play in which the pitcher (1) fields a batted ball and throws home to the catcher (2), who retires a runner advancing from third. The catcher then throws to the first baseman (3) to force out the batter.
The first baseman (3) fields a batted ball and throws to the shortstop (6) to force out a runner at second. The shortstop then throws to the pitcher (1) (who is now covering first because the first baseman was busy fielding the ball) to force out the batter.
The line between home plate and first base that begins 45 feet down the first base line and extends past first base. The rules state that if the batter-runner is in the path of a throw that originates near home plate and is outside the area created by the base line and the 45-foot line, he shall be called out if the umpire believes he interfered with the play. If he remains within the line, he cannot be called out for interference. This rule is designed to allow catchers and pitchers the ability to field bunts and throw the batter-runner out without having to worry about the batter-runner intentionally or unintentionally interfering with the throw.
This line is also used to decide whether a pickoff move is legal or a balk. If the pitcher steps with his lead foot towards the base he intends to throw to it is considered legal; the 45-foot line determines whether that step is towards the base or towards home plate. This only comes into play when the pickoff move is to the base the pitcher naturally faces, i.e. third for a right-hander or first for a lefty.
The third baseman (5) fields a batted ball and throws to the second baseman (4) to force out a runner advancing from first. The second baseman then throws to the first baseman (3) to force out the batter.