Historical Rules · 1 October 2009
Rule 107.7
Official source text from the 1 October 2009 Rules release
Each activated ability of a planeswalker has an arrow-shaped loyalty symbol in its cost. Positive loyalty symbols point upward and feature a plus sign followed by a number or an X. Negative loyalty symbols point downward and feature a minus sign followed by a number or an X. [+N] means "Put N loyalty counters on this permanent," and [-N] means "Remove N loyalty counters from this permanent."