Collisions and slides are judged by purpose, path, and possession.

How umpires check it

1. Identify which base is involved: home plate has its own collision rule, while double-play slide rules apply at the other bases.
2. For a runner, decide whether the runner stayed on a legal path and made a real attempt to reach the base or plate.
3. For a fielder, decide whether the fielder had possession of the ball, was legally fielding a throw, or blocked the runner without a right to that space.
4. Judge whether the contact was avoidable, intentional, or made for the purpose of breaking up a play rather than reaching the base.
5. Apply the proper result: an out, interference with additional outs, a safe award for illegal blocking, or no violation if the contact came from legal play.

The collision rule

A runner trying to score may not leave the direct path to the plate in order to crash into the catcher or another player covering home. The runner also may not otherwise start an avoidable collision. If the umpire judges that the runner created that contact, the runner is out even if the catcher drops the ball.

That does not mean every runner must slide. A runner can score standing up, hook around a tag, or slide. The key is that the runner is trying to touch the plate, not using the contact itself as the method of beating the tag.

What the catcher can and cannot do

A catcher who already has the ball may block the plate while making a tag. Without possession, the catcher generally cannot block the runner's pathway to home. If the catcher blocks the path without the ball and impedes the runner, the umpire can call or signal the runner safe.

There are important limits. The catcher is allowed to move into the runner's path when a throw pulls the catcher there in a legitimate attempt to field it, such as a throw that tails, bounces, or comes from a drawn-in infielder. The special home-plate blocking rule also does not apply to force plays at home, though ordinary obstruction, interference, and safety rules still matter.

What protects the runner at home

An appropriate slide usually protects the runner from being ruled to have initiated an avoidable collision. On a feet-first slide, the runner's buttocks and legs should hit the ground before contact. On a head-first slide, the runner's body should hit the ground before contact.

Umpires are also watching for signs that the runner was not really trying to score normally: lowering a shoulder, pushing through with hands or elbows, or failing to make an effort to touch the plate. Those details support a collision violation.

The bona fide slide rule

On a double-play attempt, a runner may be called for interference if the runner does not use a bona fide slide and initiates, or tries to initiate, contact with a fielder for the purpose of breaking up the double play.

A bona fide slide starts before the base, stays within reach of the base, includes an actual attempt to reach the base with a hand or foot, and allows the runner to remain on the base after the slide, except at home plate. The runner also cannot change pathway just to make contact with the fielder.

What usually gets called

• Roll blocks: rolling into the fielder instead of sliding to the base is not a bona fide slide.
• Late takeout slides: starting the slide after reaching the base is a sign the runner was targeting the fielder, not the base.
• Slides past reach of the base: a runner who cannot reach the base with a hand or foot is usually outside the protection of the rule.
• Kicking or throwing the body: raising a leg above the fielder's knee, throwing an arm, or throwing the upper body into the fielder can turn the play into interference.

Why two outs can be called

When the double-play slide rule is violated, the runner is out and the batter-runner is also out. If the runner who interfered was already out, the umpire declares out the runner on whom the defence was trying to make a play.

This is why a hard slide at second can erase a batter-runner who looked safe at first. The rule is not asking only whether the throw beat the batter-runner; it is asking whether the offence illegally prevented the defence from completing a legitimate double-play attempt.

Legal contact still happens

Baseball allows contact that comes from a legal slide or from both players making a normal play on the base. A runner who makes a bona fide slide is not called for interference merely because the slide contacts the fielder. A runner also is not responsible for contact caused by the fielder standing in, or moving into, the runner's legal path to the base.

At home plate, a catcher with the ball can make a firm tag, and a runner can slide into that tag. The violation is the avoidable collision, illegal block, or takeout action, not ordinary contact by itself.

Why local codes may be stricter

Professional-style rules use specific tests for home-plate collisions and double-play slides. Youth, school, college, and local leagues may add stricter safety language for malicious contact, flagrant collisions, mandatory avoidance, ejections, suspensions, or dead-ball penalties.

That means a play that is handled as obstruction or interference in one rule code may also bring an ejection or stronger penalty in another. For amateur games, always check the competition's own rulebook and any tournament modifications.

"The runner has to slide"

Under professional-style rules, the runner is not automatically required to slide. The runner is required to avoid illegal contact and, on a double-play attempt, must meet the bona fide slide standard if using a slide to contact the play. A legal path around a tag can be just as valid as a slide.

"The catcher can block once the throw is coming"

A coming throw does not give the catcher a blanket right to block the plate. The umpire judges whether the catcher had the ball or had to move there as part of a legitimate attempt to field the throw. If the catcher simply sets up in the runner's path without possession and blocks progress, the runner may be ruled safe.

Four quick rulings

• Runner lowers a shoulder and crashes into the catcher instead of trying to touch the plate: runner out, ball dead, other runners return to their last touched bases at the time of collision.
• Catcher receives the throw while standing in front of the plate, then tags a sliding runner: legal block with possession, runner is out if tagged before touching home.
• Throw pulls the catcher across the plate and contact follows before possession is secure: likely no blocking violation if the movement was a legitimate attempt to field the throw.
• Runner slides late past second and rolls into the shortstop to stop a throw to first: interference on the double-play attempt, with the runner and batter-runner declared out.

Source material

• MLB Official Information
• Official Baseball Rules: Rule 6.01(i) and 6.01(j)
• MLB Obstruction
• MLB Interference
• Little League Rules, Regulations, and Policies