What it does
Flanking is a combat ability found mostly on old creatures (like Knights). Whenever a creature without flanking blocks a creature with flanking, the blocking creature gets -1/-1 until end of turn. Multiple instances of flanking stack, so a creature with double flanking gives -2/-2.
The key nuance: flanking only triggers when blocking, and only matters against blockers that don't have flanking themselves. So a flanking creature blocking another flanking creature triggers nothing. The -1/-1 applies immediately on block, which can kill small blockers (1-toughness tokens) before damage is even dealt, meaning they deal no combat damage back.
In Commander, flanking is a minor but real edge for aggressive decks. It punishes chump-blocking with X/1 tokens and helps your attackers survive trades. It's irrelevant on defense, since it never triggers when you're the one blocking.













