Specifically fielder and baserunner. I see videos where fielders and baserunners bump into each other while making a play / getting to base. It seems like half the time it is an out while the other half the runner gets the base. Also the thrown ball hits the runner and they’re safe even if out of the base path. So what constitutes runner interference and fielder interference?
Note: Not asking about batter or catcher interference. Those I get.
This is the basic idea.
The fielder has the first right of way for fielding a batted ball. If he fumbles it and he or another fielder tries to get it, he no longer has right of way. (I believe, but no longer recall that if a ball glances off the pitcher other infielders still have the right of way.
If a fielder does not have the ball and is not trying to catch a thrown ball, then the runner has right of way. This type of interference by a fielder is called obstruction.
When running to first base, the batter must stay between the two lines 3 feet apart in foul territory. If a thrown ball hits him, and he’s not, that too is interference. Otherwise a thrown ball hitting a runner is not interference unless they deliberately try to get hit.
There are somewhat more complicated rules about catchers blocking the plate or trying to throw out base stealers.