Flanking



Flanking is a triggered ability that means when a creature without flanking blocks a creature with flanking, the blocking creature gets -1/-1 until end of turn.

Example
Amy is attacking Noel with a Benalish Cavalry and Noel controls a Benalish Cavalry and a Canyon Minotaur. If Noel blocks with the Canyon Minotaur, flanking will trigger and shrink the Canyon Minotaur to a 2/2 until end of turn. However, if Noel were to block with the Benalish Cavalry, flanking wouldn't trigger, as the blocking Benalish Cavalry also has flanking.

Comprehensive Rules
702.24. Flanking


 * 702.24a. Flanking is a triggered ability that triggers during the declare blockers step. (See rule 509, "Declare Blockers Step.") "Flanking" means "Whenever this creature becomes blocked by a creature without flanking, the blocking creature gets -1/-1 until end of turn."


 * 702.24b. If a creature has multiple instances of flanking, each triggers separately.