Rule 702.80a

Retrace appears on some instants and sorceries. It represents a static ability that functions while the card with retrace is in a player’s graveyard. “Retrace” means “You may cast this card from your graveyard by discarding a land card as an additional cost to cast it.” Casting a spell using its retrace ability follows the rules for paying additional costs in rules 601.2b and 601.2f–h.

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