The foul poles are tall yellow poles that mark where the foul lines meet the outfield fence. They have wire netting attached that is parallel to the outfield fence. Each field has two foul poles, one for the left foul line and one for the right foul line.
So what does fair and foul even mean? The area in between the foul lines is considered fair territory. While the area outside the foul lines is foul territory. Fair territory is the area including the first base and third base lines extending from home plate to the fences at left field and right field. Foul territory is outside the first base and third base lines extending to the fence. When the ball is in play, all defensive players, otherwise known as fielders, (except the catcher) must be in fair territory.
Whether the ball lands in fair territory or foul territory it influences the result of that play. The ball is determined to be fair or foul once it stops rolling, if a fielder touches the ball, and if it passes first base and third base while remaining in fair territory. Umpires will use the fouls lines and foul poles to easily judge if the ball is fair or foul.
Here is a list of example scenarios:
Fair or Foul Ball
The ball touches a fielder, umpire, or any other person in fair territory.
The ball goes over the fence in fair territory.
The ball touches the foul pole while traveling in fair territory.
The ball is hit directly into foul territory.
A ball hits the rubber and bounces into foul territory without being touched by a fielder.
The ball originally lands in fair territory then bounces and settles in foul territory before passing first base or third base.
The ball is touched by a fielder in foul territory.
What about the lines themselves? Are they considered fair or foul? If the ball stops rolling and remains on the foul line, it is a fair ball. The ball must be touching part of the foul line to be considered fair.
For more advanced baseball fans and statisticians, there may be situations where a fielder is in fair territory and makes a catch in foul territory and vice versa. In either case, the result is the same an out. However, it may be recorded differently.