Rule 509.1c

The defending player checks each creature he or she controls to see whether it's affected by any requirements (effects that say a creature must block, or that it must block if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of blockers is illegal. Tapped creatures and creatures withthat unpaidcan't costsblock tounless blockyou pay a cost are exempt from effects that would require them to block. Example: A player controls one creature that "blocks if able" and another creature with no abilities. An effect states, "Creatures can't be blocked except by two or more creatures." Having only the first creature block violates the restriction. Having neither creature block fulfills the restriction but not the requirement. Having both creatures block the same attacking creature fulfills both the restriction and the requirement, so that's the only option.

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The defending player checks each creature he or she controls to see whether it's affected by any requirements (effects that say a creature must block, or that it must block if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of blockers is illegal. Tapped creatures and creatures that can't block unless you pay a cost are exempt from effects that would require them to block. If a creature can't block unless a player pays a cost, that player is not required to pay that cost, even if blocking with that creature would increase the number of requirements being obeyed. Example: A player controls one creature that "blocks if able" and another creature with no abilities. An effect states "Creatures can't be blocked except by two or more creatures." Having only the first creature block violates the restriction. Having neither creature block fulfills the restriction but not the requirement. Having both creatures block the same attacking creature fulfills both the restriction and the requirement, so that's the only option.

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The defending player checks each creature he or she controls to see whether it's affected by any requirements (effects that say a creature must block, or that it must block if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of blockers is illegal. If a creature can't block unless a player pays a cost, that player is not required to pay that cost, even if blocking with that creature would increase the number of requirements being obeyed. Example: A player controls one creature that "blocks if able" and another creature with no abilities. An effect states "Creatures can't be blocked except by two or more creatures." If a creature with menace attacks that player, the player must block with both creatures. Having only the first creature block violates the restriction. Having only the first creature block violates the restriction created by menace (the attacking creature can't be blocked except by two or more creatures). Having only the second creature block violates both the menace restriction and the first creature's blocking requirement. Having neither creature block fulfills the restriction but not the requirement. Having both creatures block the same attacking creature fulfills both the restriction and the requirement, so that's the only option.

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The defending player checks each creature he or she controls to see whether it's affected by any requirements (effects that say a creature must block, or that it must block if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of blockers is illegal. If a creature can't block unless a player pays a cost, that player is not required to pay that cost, even if blocking with that creature would increase the number of requirements being obeyed. If a requirement that says a creature blocks if able during a certain turn refers to a turn with multiple combat phases, the creature blocks if able during each declare blockers step in that turn. Example: A player controls one creature that "blocks if able" and another creature with no abilities. If a creature with menace attacks that player, the player must block with both creatures. Having only the first creature block violates the restriction created by menace (the attacking creature can't be blocked except by two or more creatures). Having only the second creature block violates both the menace restriction and the first creature's blocking requirement. Having neither creature block fulfills the restriction but not the requirement.

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The defending player checks each creature hethey orcontrol she controls to see whether it's affected by any requirements (effects that say a creature must block, or that it must block if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of blockers is illegal. If a creature can't block unless a player pays a cost, that player is not required to pay that cost, even if blocking with that creature would increase the number of requirements being obeyed. If a requirement that says a creature blocks if able during a certain turn refers to a turn with multiple combat phases, the creature blocks if able during each declare blockers step in that turn. Example: A player controls one creature that "blocks if able" and another creature with no abilities. If a creature with menace attacks that player, the player must block with both creatures. Having only the first creature block violates the restriction created by menace (the attacking creature can't be blocked except by two or more creatures). Having only the second creature block violates both the menace restriction and the first creature's blocking requirement. Having neither creature block fulfills the restriction but not the requirement.

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