5.06 (a) (7.01) Occupying the Base
(1) A runner acquires the right to an unoccupied base when he touches it before he is out. He is then entitled to it until he is put out, or forced to vacate it for another runner legally entitled to that base.
Rule 5.06(a)/5.06 (c) Comment (Rule 7.01 Comment):
If a runner legally acquires title to a base, and the pitcher assumes his pitching position, the runner may not return to a previously occupied base.
(2) (7.03(a)) Two runners may not occupy a base, but if, while the ball is alive, two runners are touching a base, the following runner shall be out when tagged and the preceding runner is entitled to the base, unless Rule 5.06(b)(2) (Rule 7.03(b)) applies.