The infield fly rule is a unique rule in baseball.
It is a rule that is designed to prevent the fielding team from tricking runners and converting a double play or triple play off what should be an easy pop out.
The infield fly rule is a judgment call that umpires make based on a few factors of the specific play in question.
The odd thing about the rule is that it will reward the fielding team with an out even if they don't make a successful catch of the pop up.
It is also a dead-ball play that has one extra twist for runners.
To fully understand what the infield fly rule is, we have to break down the situations for when it applies, how umpires make the judgment call, other tidbits regarding the rule, and why it was put into the rule book in the first place.
One of the unique parts about the infield fly rule is that it is not always "in effect."
In other words, an infield fly rule can only be called in certain situations, which is determined by two factors -- how many outs there are, and how many runners are on base, and which bases they are occupying, before the play.
Both of these factors must be met before an infield fly rule can be in effect:
1. There must be less than two outs in the inning.
2. There must either be runners on first and second base, or the bases must be loaded (with runners on first, second and third base).
If both of those situations exist, then an infield fly rule may be called if the umpire judges that...
So, what "meets the criteria" of the infield fly rule?
If a batted ball is hit in the air in the infield or within easy reach of an infielder.
The umpire will make a judgment call as to whether the pop fly can be caught by any infielder, catcher, or pitcher with "ordinary effort."
That means that a line drive or popped-up bunt doesn't qualify for the infield fly rule, nor do any pop ups that would take extra effort for an infielder, the pitcher, or catcher to catch.
The infield fly rule is also only in effect when the pop up is hit in fair territory.
This means that a pop up hit in foul territory doesn't qualify for the infield fly rule, even if an infielder, pitcher, or catcher could catch it with that same "ordinary effort."
We'll get into why this is in a minute.
When the umpire judges that a batted ball meets the infield fly rule, he calls out "infield fly, batter's out."
The play is then called dead, and the hitter is out regardless of whether any fielder actually makes the catch or not.
That means that even if the ball drops to the ground, the batter is still out.
While it is a dead-ball play when the infield fly rule is called...
Even though the batter is out once the umpire calls the infield fly rule, the ball is still live, meaning that runners can advance at their own risk -- depending on what happens next.
If the fielders let the ball drop to the ground, then runners may advance as they would on a normal batted ball that hit the ground.
If the ball is caught, then the runners must tag up, just as they would if a fly ball was caught in a different situation.
In either situation, the fielding team will have to tag the runners out if they try to advance. There is no force play once the infield fly rule is called.
Also, runners are under no obligation to advance. Even if the ball drops to the ground, runners can decide to remain at the base on which they started the play.
Now that you understand what the infield fly rule is and how it's called, you may be questioning why the rule is even in place at all -- or maybe you have already figured it out yourself.
The idea behind the rule is to prevent the fielding team from faking runners out on a routine pop up on the infield, only to let the ball drop to the ground and turn a double play or triple play instead.
If the infield fly rule was not in place, then runners would be faced with two tough choices when pop ups were hit on the infield:
They could stay close to their bag. If the infielder decided to drop the ball on purpose (or miss it by mistake), then they'd have to hurry to try to advance to the next bag before the force out was applied.
They could linger off their bag and head toward the next base. If the infielder caught the easy pop up, though, the runner would have to hurry to make it back to their original bag so they weren't doubled up.
The infield fly rule is in place to even the playing field for both the fielding and hitting team.
Without the rule, the hitting team would be at a severe disadvantage and would be very susceptible to having the fielding team turn a double play or even triple play against them.
The infield fly rule, then, awards the fielding team with an out for successfully getting the batter to pop up on the infield, while not penalizing the runners to an extreme level.