+1/+1 counter

+1/+1 counters are the most common counter type in Magic. They were introduced in Alpha. A +1/+1 counter on a creature adds one to its power and toughness. Creatures using +1/+1 counters were among the most popular in the early history of the game.

For ease of comprehension, blocks tend to focus on only a single type of counter on creatures, and +1/+1 counters are the default choice. When +1/+1 counters are absent from a block, they are most commonly replaced by their inverse, -1/-1 counters. Occasionally, thematic counters are used instead, such as the ki and divinity counters of Kamigawa block.

Charge counters serve as the equivalent default counter for noncreature permanents.

As a further concession to mitigate complexity, +1/+1 counters and -1/-1 counters are the only power and toughness altering counters used in new products. Additionally, they are never used in the same block, to minimize confusion in limited play. A small handful of cards from the earliest sets still use other, obsolete counters.

Cancellation with -1/-1 counters
When both +1/+1 and -1/-1 counters are present on the same creature, they are immediately removed in matched pairs until only one or the other remains.

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Mechanics
+1/+1 counters are a component of many ability words, keyword abilities, and keyword actions. Their ubiquity is largely due to their lack of flavor, and enables a high level of compatibility between thematically unrelated mechanics.

They are used by the following mechanics:
 * Keyword abilities
 * Adapt
 * Amass
 * Amplify
 * Awaken
 * Bloodthirst
 * Dethrone
 * Devour
 * Evolve
 * Fabricate
 * Graft
 * Mentor
 * Modular
 * Outlast
 * Reinforce
 * Riot
 * Scavenge
 * Sunburst (on creatures)
 * Undying
 * Unleash
 * Keyword actions
 * Adapt
 * Bolster
 * Explore
 * Monstrosity
 * Support
 * Ability words
 * Converge (on creatures)
 * Heroic (not general)
 * Other mechanics
 * Khabál ability