Similarly, there are no regulations at all concerning the dimensions of foul territory. Thus a foul fly ball may be entirely out of play in a park with little space between the foul lines and the stands, but a foulout in a park with more expansive foul ground. A fence in foul territory that is close to the outfield line will tend to direct balls that strike it back toward the fielders, while one that is farther away may actually prompt more collisions, as outfielders run full speed to field balls deep in the corner. These variations can make the difference between a double and a triple or inside-the-park home run. The surface of the field is also unregulated. While the adjacent image shows a traditional field surfacing arrangement (and the one used by virtually all MLB teams with naturally surfaced fields), teams are free to decide what areas will be grassed or bare. Some fields—including several in MLB—use an artificial surface, such as AstroTurf. Surface variations can have a significant effect on how ground balls behave and are fielded as well as on baserunning. Similarly, the presence of a roof (seven major league teams play in stadiums with permanent or retractable roofs) can greatly affect how fly balls are played. While football and soccer players deal with similar variations of field surface and stadium covering, the size and shape of their fields are much more standardized. The area out-of-bounds on a football or soccer field does not affect play the way foul territory in baseball does, so variations in that regard are largely insignificant.