Which base can there be a forced run?

Answer 1

When a runner is on a base that a batter or another base runner is required to run to, the former is forced to run to the next base. Two examples and a counter-example:

1) A runner begins the play on first base, and the ball is batted fair. Since the batter is required to go to first base, the runner that began on that base is forced to go to second base, and remains required to do so until the batter is out.

2) Runners begin the play on first base and on second base, and the ball is batted fair. As noted in example (1), the runner on first base is forced to go to second. Thus, the runner that began on second is now forced to go to third base. If either the batter or the runner that began on first base become out, then this requirement is cancelled.

3) A runner begins the play on third base, and the ball is batted fair. The runner MAY advance from third towards home, but is not FORCED to do so. That's because the batter is only required to run to first, and there is no requirement that the runner on third leave his base.

Answer 2

3rd base