Foul Lines and Foul Poles
Both are fair, which makes them two of the worst named things in sport.
What it actually is. The foul lines run from the back corners of home plate through the outside edges of first and third and out to the fence. The foul poles stand where those lines meet the outfield wall. Both the lines and the poles are in fair territory, so a ball that lands on the chalk is fair and a ball that hits the pole is a home run.
What somebody who knows the game gets out of it. The lines are also the reason the corner outfield positions are different jobs. A right fielder has to play the line to protect against a double while covering the largest gap in most parks, which is a genuine trade off and is exactly what no doubles defense is about in the late innings. The angles at the corners vary far more between parks than center field does, which is why the corner spots are the most park dependent defensive positions to evaluate.
The numbers worth knowing. Foul poles are typically 30 to 45 feet tall and the lines are 90 feet to the bag and then out to a fence typically 310 to 350 feet away down the line.
What people get wrong about it. Fair or foul on a fly ball is judged by where the ball is when it passes the pole or lands, not by where it is when it lands in the seats behind. A ball that is fair as it passes the pole and hooks foul afterwards is a home run, which is why the umpire's eyes are on the pole rather than on the ball's final resting place.
At Little League and travel ball. Many youth fields have short or no foul poles and rely on the umpire's judgment out to the fence, and plenty have no fence at all. Coaches should expect the line ruling to be inconsistent and should teach outfielders to play the ball rather than the call.
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