Posterior.
OBR 5.09(b)(9)

Passing a preceding runner

A runner who passes a teammate ahead of him on the bases is out, full stop. The ball stays live, and it does not matter whether the pass was intentional — losing track of the runner in front is one of baseball's self-inflicted outs.

Key points
  • Only the trailing runner is out; the preceding runner is unaffected.
  • Most common on deep fly balls: the lead runner holds up to tag, the trailing runner is watching the ball.
  • On a home run, passing a teammate turns the homer into an out for the batter-runner — and it has ended games.
  • The pass is judged the moment the trailing runner goes by, even by a step.
In a game

Runner on first thinks the liner is caught and retreats; the batter-runner rounds first with his head down and blows past him.

Ruling · Batter-runner is out the instant he passes. If the ball dropped, the lead runner still has to advance.

Why it moves the scoreboard

Converts extra-base hits — occasionally home runs — into outs, and has flipped the scoring of entire innings.

Related rules