Historical Rules · 15 July 2011
Rule 702.64a
Official source text from the 15 July 2011 Rules release
Delve is a static ability that functions while the spell that has delve is on the stack. "Delve" means "As an additional cost to cast this spell, you may exile any number of cards from your graveyard. Each card exiled this way reduces the cost to cast this spell by {1}." Using the delve ability follows the rules for paying additional costs in rules 601.2b and 601.2eg.