Part of a baseball field, a foul pole is the tall, narrow, usually yellow pole that stands at the very end of both foul lines in every ballpark. Its purpose is to define the fair/foul boundary when the ball is in the air. If the ball were to hit the foul pole, it would be a fair ball because the poles themselves are in fair territory. In the modern game of baseball, the ball is fair if it travels over the wall in fair territory, but this was not always the case. Prior to 1920, the ball had to land in fair territory on the other side of the wall in order to be considered fair.
Most Major League ballparks have very high foul poles to help umpires make the correct call on fly balls hit down the foul line. However, there are some parks that have shorter poles or no pole at all, as is the case in San Diego's Petco Park, which has a yellow line drawn up the side of a historic building that borders the park on one side. Although typically yellow, it is only important that the pole be easily visible and not white, so as not to be confused with the ball.