SRD - Combat - MOVEMENT, POSITION, AND DISTANCE
Moving through a Square
Friend: You can move through a square occupied by a friendly character, unless you are charging. When you move through a square occupied by a friendly character, that character doesn’t provide you with cover.
Ending Your Movement: You can’t end your movement in the same square as another creature unless it is helpless.
Very Small Creature: A Fine, Diminutive, or Tiny creature can move into or through an occupied square. The creature provokes attacks of opportunity when doing so.
Square Occupied by Creature Three Sizes Larger or Smaller: Any creature can move through a square occupied by a creature three size categories larger than it is.
A big creature can move through a square occupied by a creature three size categories smaller than it is.
BIG AND LITTLE CREATURES IN COMBAT
Creatures smaller than Small or larger than Medium have special rules relating to position.
Tiny, Diminutive, and Fine Creatures: Very small creatures take up less than 1 square of space. This means that more than one such creature can fit into a single square. A Tiny creature typically occupies a space only 2-1/2 feet across, so four can fit into a single square. Twenty-five Diminutive creatures or 100 Fine creatures can fit into a single square. Creatures that take up less than 1 square of space typically have a natural reach of 0 feet, meaning they can’t reach into adjacent squares. They must enter an opponent’s square to attack in melee. This provokes an attack of opportunity from the opponent. You can attack into your own square if you need to, so you can attack such creatures normally. Since they have no natural reach, they do not threaten the squares around them. You can move past them without provoking attacks of opportunity. They also can’t flank an enemy.
