In baseball, the baseline essentially is the path of a runner who is attempting to reach a base safely. There are white lines on a baseball field that go from near home plate to first base and third base and then extend past those bases to the outfield fence, as shown in the photo below. Although these lines are often referred to as the first-base line and the third-base line, they technically are not baselines, they are foul lines. Baselines are not marked on the field at all, and they come into consideration only when a defensive player is attempting to tag a runner to get him out. If a runner goes more than 3 feet (about 0.9 m) out of his baseline to avoid a tag, he or she is declared out.
Running the Bases
Base runners, especially when they are passing more than one base during a single play, typically do not run in a straight line from one base to the next. A player who is going from home plate to second base, for example, typically takes a wide turn and "rounds" first base instead of turning at a 90-degree angle, which would be impossible to do at full speed. A runner might also need to avoid an opponent who is attempting to field a batted ball and would therefore stray from an imaginary straight line between the bases. All of these deviating paths to the next base are legal in most cases.
The only time that a baseline must be determined is when a defensive player tries to tag out a runner. At that point, the baseline becomes an imaginary straight line between the runner's current location and the base that he or she is trying to reach. The runner must not stray more than 3 feet (about 0.9 m) from this imaginary line to avoid the tag, or he or she will be called out. If the runner strays from his or her baseline for any other reason, or if the defensive player does not try to make a tag, the runner will not be called out.
The 3-Foot Line
A base runner is similarly constrained while running from home plate to first base. The last half of the distance from home to first often includes a line that runs parallel to the first-base foul line and is 3 feet (about 0.9 m) over in foul territory. This line, called the 3-foot line, can be seen in the photo below. When this line is not marked on the field, it is an imaginary line.
A batter who is running to first base will be called out if he or she does not stay between the foul line and the 3-foot line — other than to step on first base — and thereby interferes with a player who is trying to catch a throw to first base. Any interference is legal if the runner stays between these lines. The runner also is permitted to run outside of this area if no interference occurs. If the batter leaves this area and is hit by a thrown or batted ball while in fair territory, however, he or she will be called out.
The foul lines on a baseball field extend from the batter's box near home plate, over the outside edges of first base and third base, then to the outfield fence. These white lines typically are made using powdered chalk or mineral lime, although white paint usually is used on grass and artificial turf. There is no standard width for these lines, but most of them are about 2 to 3 inches (about 5 to 7.6 cm) wide. Although they are called foul lines, they're actually in fair territory — the baseball must be completely outside of the line to be in foul territory.