text: When using improvise to cast a spell with {X} in its mana cost, first choose the value for X. That choice, plus any cost increases or decreases, will determine the spell’s total cost. Then you can tap artifacts you control to help pay that cost. For example, if you cast Whir of Invention (a spell with improvise and mana cost {X}{U}{U}{U}) and choose X to be 3, the total cost is {3}{U}{U}{U}. If you tap two artifacts, you’ll have to pay {1}{U}{U}{U}.