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An inside-the-park home run occurs when a batter hits the ball into play and is able to circle the bases before the fielders can put him out. Unlike with an outside-the-park home run, the batter-runner and all preceding runners are liable to be put out by the defensive team at any time while running the bases. This can only happen if the ball does not leave the ballfield.

In the early days of baseball, outfields were relatively much more spacious, reducing the likelihood of an over-the-fence home run, while increasing the likelihood of an inside-the-park home run, as a ball getting past an outfielder had more distance that it could roll before a fielder could track it down.

With outfields much less spacious and more uniformly designed than in the game's early days, inside-the-park home runs are now a rarity. They are usually the result of a ball being hit by a very fast runner, coupled with an outfielder either misjudging the flight of the ball (e.g., diving and missing) or the ball taking an unexpected bounce. Either way, this sends the ball into open space in the outfield and thereby allows the batter-runner to circle the bases before the defensive team can put him out. The speed of the runner is crucial as even triples are relatively rare in most modern ballparks.

If any defensive play on an inside-the-park home run is labeled an error by the official scorer, a home run is not scored; instead, it is scored as a single, double, etc., and the batter-runner and any applicable preceding runners are said to have taken all additional bases on error. All runs scored on such a play, however, still count.

An example of an unexpected bounce occurred during the 2007 Major League Baseball All-Star Game at AT&T Park in San Francisco on July 10, 2007. Ichiro Suzuki of the American League team hit a fly ball off the right-center field wall, which caromed in the opposite direction from where National League right fielder Ken Griffey, Jr. was expecting it to go. By the time the ball was relayed, Ichiro had already crossed the plate standing up. This was the first inside-the-park home run in All-Star Game history, and led to Suzuki being named the game's Most Valuable Player.
Specific situation home runs

These types of home runs are characterized by the specific game situation in which they occur, and can theoretically occur on either an outside-the-park or inside-the-park home run.
Grand slam
Main article: Grand slam (baseball)

Home runs are often characterized by the number of runners on base at the time, if any. A home run hit with the bases empty is seldom called a "one-run homer", but rather a "solo" homer. With one or two runners on base, the home runs are usually called "two-run homers" or "three-run homers". The term "four-run homer" is seldom used. Instead, it is nearly always called a "grand slam".

A grand slam occurs when the bases are "loaded" (that is, there are base runners standing at first, second, and third base) and the batter hits a home run. According to The Dickson Baseball Dictionary, the term originated in the card game of contract bridge. An inside-the-park grand slam is a grand slam without the ball leaving the field, and it is very rare, due to the relative rarity of loading the bases along with the significant rarity (nowadays) of inside-the-park home runs.

On July 25, 1956 Roberto Clemente became the only MLB player to have ever scored a walk-off inside-the-park grand slam in a 9–8 Pittsburgh Pirates win over the Chicago Cubs, at Forbes Field.

On April 23, 1999, Fernando Tatís made history by hitting two grand slams in one inning, both against Chan Ho Park of the Los Angeles Dodgers. With this feat, Tatís also set a Major-League record with 8 RBI in one inning.

On July 29, 2003 against the Texas Rangers, Bill Mueller of the Boston Red Sox became the only player in major league history to hit two grand slams in one game from opposite sides of the plate. In fact, he hit three home runs in that game, and his two grand slams were in consecutive at-bats.

On August 25, 2011 the New York Yankees became the first team to hit three grand slams in one game vs the Oakland A's. The Yankees eventually went on to win the game 22–9, after trailing 7-1.
Walk-off home run
Main article: Walk-off home run

A walk-off home run is a home run hit by the home team in the bottom of the ninth inning, any extra inning, or other scheduled final inning, which gives the home team the lead and thereby ends the game. The term is attributed to Hall of Fame relief pitcher Dennis Eckersley,[4] so named because after the run is scored, the players can "walk off" the field.

Two World Series have ended via the "walk-off" home run. The first was the 1960 World Series when Bill Mazeroski of the Pittsburgh Pirates hit a 9th inning solo home run in the 7th game of the series off New York Yankees pitcher Ralph Terry to give the Pirates the World Championship. The second time was the 1993 World Series when Joe Carter of the Toronto Blue Jays hit a 9th inning 3-run home run off Philadelphia Phillies pitcher Mitch Williams in Game 6 of the series, to help the Toronto Blue Jays capture their second World Series Championship in a row.

Such a home run can also be called a "sudden death" or "sudden victory" home run. That usage has lessened as "walk-off home run" has gained favor. Along with Mazeroski's 1960 shot, the most famous walk-off or sudden-death homer would probably be the "Shot Heard 'Round the World" hit by Bobby Thomson to win the 1951 National League pennant for the New York Giants.

A walk-off home run over the fence is an exception to baseball's one run rule. Normally if the home team is tied or behind in the ninth or extra innings the game ends as soon as the home team scores enough runs to achieve a lead. If the home team has two outs in the inning, the game will officially end either the moment the batter successfully reaches 1st base or the moment the runner touches home plate - whichever happens last. However, this is superseded by the "ground rule", which provides automatic doubles (when a ball-in-play hits the ground first then leaves the playing field) and home runs (when a ball-in-play leaves the playing field without ever touching the ground). In the latter case, all base runners including the batter are allowed to cross the plate.
Lead-off home run