118.9. Some spells have alternative costs. An alternative cost is a cost listed in a spell's text, or applied to it from another effect, that its controller may pay rather than paying the spell's mana cost. Alternative costs are usually phrased, "You may [action] rather than pay [this object's] mana cost," or "You may cast [this object] without paying its mana cost." Note that some alternative costs are listed in keywords; see rule 702.
118.9a. Only one alternative cost can be applied to any one spell as it's being cast. The controller of the spell announces their intentions to pay that cost as described in rule 601.2b.