The battleline is the ordered line of creatures a player controls in play.
The far left and right edges of a battleline are the flanks. When a creature is in one of these positions, it is “on the flank” and is also known as a “flank creature.” When a battleline consists of exactly one creature, that creature is on both flanks.
Creatures enter play on the flank of their controller's battleline (the active player chooses which flank).
Each time a creature leaves play, the battleline shifts inward to close the gap.