212.9f.

Each planeswalker has a number of activated abilities. A player may play an activated ability of a planeswalker only during a main phase of his or her turn, when he or she has priority and the stack is empty, and only if none of its activated abilities have been played that turn. The cost to play an activated ability of a planeswalker is to put on or remove from that planeswalker a certain number of loyalty counters, as shown by the loyalty symbol in the ability's cost. An ability with a negative loyalty cost can't be played unless the planeswalker has at least that many loyalty counters on it.