A pinch-runner is a substitute. A courtesy runner exists only if the rulebook allows it.

How to identify the runner change

1. Start with the competition rule set, because courtesy-runner rules vary sharply outside professional-style baseball.
2. Ask whether the offensive team is making a normal substitution or invoking a specific courtesy-runner rule.
3. If it is a pinch-runner, record the original runner as removed and place the substitute in that batting-order spot.
4. If it is a courtesy runner, check who is eligible, when the request must be made, and whether the same player can be used again.
5. Before the next half-inning or plate appearance, confirm which player is legally in the lineup spot and which player must return to the base, field, bench, or batting order.

A pinch-runner replaces a runner in the game

A pinch-runner is a substitute used for baserunning. The manager sends a player from the bench to take the place of a runner who is already on base. In professional-style rules, that move removes the original runner from the game immediately.

The pinch-runner does not create a new lineup spot. The pinch-runner inherits the replaced player's batting-order position, just as a pinch-hitter, defensive replacement, or other substitute would. After running, the pinch-runner may stay in the game on defense, bat later in that same lineup spot, or be replaced by another legal substitute.

A courtesy runner is not a general baseball rule

A courtesy runner is a temporary runner allowed only because a particular league, school association, tournament, or local rule says so. The purpose is usually practical: to speed up play, protect pitchers or catchers, manage participation rules, or handle a narrow injury or equipment situation.

Because it is a special rule, there is no universal baseball answer for who may courtesy-run. Some competitions restrict courtesy runners to pitchers or catchers. Some require a player not currently in the lineup. Some use the last completed out. Some bar the same player from courtesy-running more than once in an inning or for both battery positions. Those details must be checked in the competition rules, not guessed from MLB practice.

Courtesy runners are prohibited in professional-style play

Under the Official Baseball Rules used by MLB-style play, a player whose name is already in the batting order may not become a substitute runner for a teammate. A player already removed from the game may not return as a courtesy runner. A player from the bench who is used as a runner is treated as a substitute player.

That means the ordinary professional-style answer is simple: if a team wants a different runner, it uses a pinch-runner and accepts the substitution consequences. The original runner is done for the game unless a different rule set specifically allows re-entry.

Runner changes usually happen while the ball is dead

A pinch-runner is normally reported while the ball is dead, after the original player has reached base and before play resumes. The manager should notify the plate umpire, and the scorer records the substitution in the replaced player's lineup spot.

Courtesy-runner timing depends on the rule that creates it. Many amateur rules require the courtesy runner to be announced before the next pitch or play, and some limit when a courtesy runner may enter for the same player. If the rule has a deadline, missing that deadline can make the runner illegal or force the team to use a normal substitution instead.

The biggest difference is whether the original player leaves

With a pinch-runner, the original runner has been substituted out. In MLB-style baseball, that player cannot return to run, field, bat, pitch, catch, or serve as a courtesy runner later. The pinch-runner now owns that lineup spot unless replaced.

With a valid courtesy runner, the original player normally remains in the game and returns under the courtesy-runner rule. For example, if a local rule permits a courtesy runner for the catcher, the catcher may be allowed to return on defense the next half-inning without the courtesy runner taking the catcher's batting-order spot. The exact return procedure depends on the local rule.

An injured runner may still require a substitution

If a runner is injured and cannot continue, professional-style rules do not turn that into a free courtesy-runner situation. The team uses a legal substitute, and the injured player is removed from the game. If no legal substitute is available, the competition's shortage, forfeit, or injury rules control what happens next.

Some amateur competitions have separate injury, blood, concussion, or temporary-replacement procedures. Those procedures may be more flexible than professional substitution rules, but they are not automatic. Officials apply the written competition rule and any safety policy for that event.

How umpires enforce the difference

Umpires first decide what the offensive team actually requested. If the manager reports a pinch-runner, the umpire records a substitution. If the manager requests a courtesy runner, the umpire checks whether that game has a courtesy-runner rule and whether the proposed runner is eligible.

If an ineligible courtesy runner is discovered, the remedy depends on the rule set and timing. In many games, the umpire will correct the illegal player before play resumes. If play has already resumed, the competition's illegal-substitution, illegal-runner, or protest rule determines whether the play stands, whether the runner is removed, and whether additional penalties apply.

Misunderstandings to avoid

• "A pinch-runner is just temporary" is wrong under MLB-style rules. The original runner has been replaced.
• "Courtesy runners are always allowed for catchers" is wrong. That is common in some amateur settings but not a universal baseball rule.
• "Any bench player can courtesy-run" is incomplete. The rule may restrict eligibility, repeat use, and whether the runner has already appeared in the game.
• "A player already in the lineup can run for a teammate in MLB-style play" is wrong. That is the courtesy-runner practice the professional rule is designed to prevent.
• "An injured player always gets a free runner" is wrong. Injury replacement is still governed by the rule set being used.
• "The umpire can choose a fair runner" is wrong. Officials apply the lineup and the competition rule; they do not invent a replacement system during the game.

What changes the ruling

• MLB-style game, runner on first is replaced by a faster bench player: that bench player is a pinch-runner. The original runner is out of the game.
• High school or youth rule allows a courtesy runner for the catcher: the team may use one only if the proposed runner satisfies that rule's eligibility limits.
• Player already batting fifth tries to run for the player batting eighth in a professional-style game: that is not allowed as a courtesy runner.
• Courtesy runner later comes to bat in the wrong spot: officials must check whether that player was only a temporary runner or had also entered as a substitute.
• Pinch-runner scores and then stays in left field: that is legal if the defensive move is properly reported; the pinch-runner remains in the same batting-order spot.

Source material

• MLB Official Baseball Rules: Rule 5.10
• MLB Official Information
• MLB Automatic Runner Glossary
• Little League Rules, Regulations, and Policies