The grass line, where the dirt of the infield ends and the grass of the outfield begins, has no special significance to the rules of the game (except in Double-A Minor League Baseball where all infielders must be on the infield dirt when the pitch is thrown as part of an experimental rule for the 2021 season), but it can influence the outcome of a game. Dirt running paths between the bases (and, at one time and still in some parks, between the pitcher and the catcher) have existed since the beginning of the game, although they were not mentioned in the rule books until around 1950, and their specifications are flexible. In addition to providing a running path, the grass lines act as a visual aid so that players, umpires and fans may better judge distance from the center of the diamond. Occasionally the ball may take a tricky bounce off the dirt area or the edge between the dirt and the grass. Multiple World Series championships (including 1924, 1960 and 1986) have been decided or heavily influenced by erratic hops of ground balls.
In artificial turf stadiums, infield dirt was originally only placed in three five-sided areas around the bases and in two circles around the pitcher's and batting areas, which are referred to as "sliding pits". In this configuration, the "grass line" is usually designated with a white arc. This setup first appeared at Cincinnati's Riverfront Stadium upon its opening in 1970. Among Major League Baseball fields, Rogers Centre was the last stadium to maintain this type of configuration and was reconfigured with a full dirt infield starting in the 2016 MLB season.
In some college baseball parks with artificial turf fields, the entire field (along with possibly the pitcher's mound) is made up of turf, with parts of the field mainly containing dirt instead merely being clay-colored turf.