421. Handling “Infinite” Loops
421.1. Occasionally the game can get into a state in which a set of actions could be repeated forever.
These rules (sometimes called the “infinity rules”) govern how to break such loops.
421.2. If the loop contains one or more optional actions and one player controls them all, that player
chooses a number. The loop is treated as repeating that many times or until another player
intervenes, whichever comes first.
421.3. If a loop contains optional actions controlled by two players and actions by both of those players
are required to continue the loop, the active player (or, if the active player is not involved, the first
involved player after the active player in turn order) chooses a number. The other player then has
two choices. He or she can choose a lower number, in which case the loop continues that number of
times plus whatever fraction is necessary for the first player to “have the last word.” Or he or she
can agree to the number the first player chose, in which case the loop continues that number of
times plus whatever fraction is necessary for the second player to “have the last word.” (Note that
either fraction may be zero.) This sequence of choices is extended to all applicable players if there
are more than two players involved.
Example: In a two-player game, one player controls a creature with the ability “{0}: [This
creature] gains flying,” and another player controls a permanent with the ability “{0}:
Target creature loses flying.” The “infinity rule” ensures that regardless of which player
initiated the gain/lose flying ability, the nonactive player will always have the final choice
and therefore be able to determine whether the creature has flying. (Note that this assumes
that the first player attempted to give the creature flying at least once.)
421.4. If the loop contains only mandatory actions, the game ends in a draw. (See rule 102.4b.)
421.5. If the loop contains more than one set of optional, independent actions, each controlled by
different players, then the active player (or, if the active player is not involved, the first involved
player after the active player in turn order) chooses a number for his or her set of actions. Knowing
that number, the remaining players, in turn order, each choose a number for his or her sets of
actions. It can be higher, lower, or the same. Then each set of actions occurs the appropriate number
of times.
421.6. If the loop contains an effect that says “[
X] unless [Y],” where [
X] and [Y] are each actions, no
player can be forced to perform [Y] to break the lo