Batting out of order is an appeal rule.

How umpires sort it out

1. Check the official lineup card, including any reported substitutions.
2. Find the proper batter: the player whose spot follows the last legal or legalized batter.
3. If the improper batter is still batting, replace that batter with the proper batter and keep the current ball-strike count.
4. If the improper batter has completed the plate appearance, wait for a defensive appeal.
5. If the appeal is timely, call the proper batter out and reset runners according to what the rules allow.
6. If the appeal is late, legalize the improper batter's result and continue with the next spot after that batter.

The lineup card controls the order

The batting order is the official sequence submitted before the game and modified only through legal substitutions. A team bats out of order when someone bats instead of the player whose turn it actually is.

The rule is not about whether the wrong player is eligible to play baseball. It is about sequence. A legal player can still be an improper batter if that player's turn has not arrived.

The proper batter can take over

If the mistake is noticed while the improper batter is still at bat, the fix is simple. The proper batter replaces the improper batter in the box and inherits the existing count. There is no out merely because the wrong player started the plate appearance.

This can happen after either team notices. The important point is timing: the improper batter has not yet become a runner and has not been put out.

After the wrong batter finishes

Once the improper batter becomes a runner or is put out, the defence must appeal before the first pitch to the next batter of either team, or before another play or attempted play. If the appeal is made in time, the proper batter - the player who should have batted - is called out.

The improper batter's completed result is wiped out for lineup purposes. Any advance or score caused by that batter putting the ball in play or reaching first on a hit, error, walk, hit by pitch, or similar result is nullified.

Some runner movement still counts

Not every advance during the wrong batter's plate appearance disappears. If a runner advanced on a stolen base, balk, wild pitch, or passed ball while the improper batter was at bat, that advance remains legal because it was not caused by the improper batter completing the turn.

That distinction is why umpires separate what happened during the plate appearance from what happened because of the completed plate appearance.

The next pitch can legalize it

If the defence does not appeal before the next pitch, play, or attempted play, the improper batter becomes the proper batter for that completed turn. The result stands, even if everyone later realizes the lineup was wrong.

After legalization, the batting order does not rewind to the skipped player. It continues with the player whose name follows the legalized improper batter in the official order.

The umpire does not announce the mistake

Under professional-style rules, the umpire is not supposed to alert either team that the wrong batter is in the box. Batting out of order is built around team responsibility: the offence must send up the right batter, and the defence must make a timely appeal if it wants the penalty.

Once an appeal is made, the umpire uses the official lineup and the timing of the appeal to decide whether to correct the count, call the proper batter out, or legalize the improper turn.

Replacements inherit lineup spots

A substitute takes the batting-order spot of the player replaced. That means batting out of order questions often start with a lineup-card check: who legally entered, who left, and which spot did the substitute occupy?

Some amateur and youth rules allow re-entry or other substitution variations. Those rules can change who is legally in a spot, but they do not change the basic need to identify the proper batter under the rule set being used.

Misunderstandings to avoid

• "The wrong batter is automatically out" is wrong. If the mistake is caught during the plate appearance, the proper batter simply takes over the count.
• "The batter who hit out of turn is the one called out" is usually wrong under professional-style rules. On a timely appeal after a completed turn, the proper batter who failed to bat is called out.
• "Any appeal later in the inning works" is wrong. A pitch, play, or attempted play after the improper turn can make the result legal.
• "All runner movement disappears" is incomplete. Steals, balks, wild pitches, and passed balls during the improper plate appearance can still count.
• "The umpire should warn the teams" is wrong in professional-style rules. Teams are expected to monitor the order themselves.

What changes the ruling

• Wrong batter has a 2-1 count: the proper batter enters with a 2-1 count if the mistake is noticed before the plate appearance ends.
• Wrong batter singles and the defence appeals immediately: the proper batter is out, the hit is nullified, and runners return unless their advance was legal for another reason.
• Wrong batter singles and the next batter receives a pitch: the single is legalized, and the next proper batter is the player after the legalized batter.
• Runner steals second while the wrong batter is hitting, then the wrong batter grounds out: on a timely appeal, the proper batter is out, but the steal can still stand.

Source material

• MLB Official Baseball Rules: Rule 6.03(b)
• MLB Official Information
• MLB Rules Glossary
• Little League Rules, Regulations, and Policies