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.
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.