A Base is one of the three spots in the infield that a baserunner must touch in order to score a run. A baserunner is also allowed to stop at a base if he cannot continue all the way to home plate on the play.
The three bases are numbered from first to third, running counter-clockwise from home plate, and must always be touched in order. With home plate, they form a perfect square shape known as the diamond.
- First base is located 90 feet from home plate, at a 45 degree angle from the line between home plate and the center of the pitcher's mound, on the right foul line. It is the only base that a baserunner is allowed to overrun without being put out, as long as he turns to the right (i.e. towards foul territory) after having passed the base, and returns promptly to it. This is only allowed on batted balls; on all other plays, such as pick-off attempts or a runner returning to first base after a fly ball has been caught, he must physically touch the base or risk being tagged out.
- Second base is located 90 feet from first base, on a straight line with home plate and the center of the pitcher's mound.
- Third base is located 90 feet away from both second base and home plate, with the lines between second and third base and third base and home plate intersecting at a 90 degree angle. It is located on the left foul line. Note that the pitching mound is not on a straight line between first base and third base, as the pitching rubber is located 60' 6" from home plate; it would need to be 63' 6" away to be on a straight line with the two corner bases.
Like the foul lines, the three bases are in fair territory. However, before 1887, first and third base were centered on the base line, but were moved to be entirely within fair territory to facilitate the work of umpires. As a result, any batted ball that hits a base is fair, even if it bounces into foul territory or out of play as a result. But, contrary to first and third base, second base was never moved, so its center remains at the intersection of the two 90-foot lines, putting it slightly out of alignment with the corner bases. In 2022, this will be rectified in the minor leagues, with the result that second base will be moved closer to home plate by a few inches. The change is likely to come into the major leagues as well at some point.
According to Official Baseball Rule 1.06, First, second and third bases shall be marked by white canvas bags, securely attached to the ground as indicated in Diagram 2. The first and third base bags shall be entirely within the infield. The second base bag shall be centered on second base. The bags shall be 15 inches square, not less than three nor more than five inches thick, and filled with soft material. In 2019, Major League Baseball reached an agreement with the Atlantic League to test out larger bases, of 18 inches square. This was seen as a way of averting some collisions at first base and facilitating stolen bases. In 2021, this rule was adopted in AAA, and a different material was tested, one that would be less slippery under wet conditions, another move aimed at preventing some injuries. Initial returns were positive and the rule was adopted as part of other changes to the rules introduced for the 2023 Major League Baseball season.
- Chris Bumbaca: "Agent Scott Boras blasts MLB for wet bases after Kris Bryant injury", USA Today, September 24, 2019. 
- Anthony Castrovince: "When is 90 feet not 90 feet but still 90 feet? Let us explain", mlb.com, March 24, 2023.