A base on balls (BB) is credited to a batter and against a pitcher in baseball statistics when a batter receives four pitches that the umpire calls balls. It is better known as a walk. The base on balls is defined in Section 2.00 of baseball's Official Rules, and further detail is given in 6.08(a). It is called a "walk" because the batter is then entitled to walk to first base, or more specifically (as defined in the rules of baseball) he is "entitled to first base without liability to be put out." However, the term "base on balls" is used in official context because it is considered a faux pas for a professional player to walk to first base (as opposed to physically running), and also to distinguish from the other manners in which a batter can be awarded first base without liability to be put out (e.g., hit by pitch, catcher's interference).
A batter who draws a base on balls is commonly said to have been "walked" by the pitcher. When the batter is walked, runners advance one base without liability to be put out only if forced to vacate their base to allow the batter to take first base. If a batter draws a walk with the bases loaded, all preceding runners are forced to advance, including the runner on third base who is forced to home plate to score a run, and the batter is credited with an RBI per rule 10.04.
Receiving a base on balls does not count as a hit or an at bat for a batter but does count as a time on base and a plate appearance. Therefore, a base on balls does not increase nor decrease a player's batting average, but it does increase his on-base percentage.
A hit by pitch is not counted statistically as a walk, though the effect is mostly the same, with the batter receiving a free pass to first base. The one exception is that on a HBP, the ball is dead and any runners who were stealing on the play must return to their original base unless forced to the next base anyways.
Intentional base on balls
A subset of the base on balls, an intentional base on balls (IBB) or intentional walk is when the pitcher deliberately pitches the ball away from the batter in order to issue a base on balls. As with any other walk, an intentional walk entitles the batter to first base without liability to be put out, and entitles any runners to advance if forced. Intentional walks are a strategic defensive maneuver, usually done to bypass one hitter for one the defensive team believes is less likely to initiate a run-scoring play (e.g., a home run, sacrifice fly, or RBI base hit), or to set up a double play or force out situation for the next batter. They do carry an inherent risk, however, as they give the offensive team another runner on base, without any effort on their part, who could potentially score a run.
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