Historical Rules · 10 August 2018
Rule 121.1a
Official source text from the 10 August 2018 Rules release
A +X/+Y counter on a creature or on a creature card in a zone other than the battlefield, where X and Y are numbers, adds X to that object’s power and Y to that object’s toughness. Similarly, -X/-Y counters subtract from power and toughness. See rule 613.3.