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+1/+1 counters give −1/−1 and −1/−1 counters give +1/+1. (They're still +1/+1 and −1/−1 counters. The effects are just backwards.)
Read from the rules text. A card that enables a keyword makes or is the thing; a card that rewards it gets better when you have it. Two cards interact when one enables what the other rewards.
