Chord of Calling

Because convoke isn't an alternative cost, it can be used in conjunction with alternative costs.

When using convoke to cast a spell with X generic mana 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 creatures you control to help pay that cost. For example, if you cast Chord of Calling (a spell with convoke and mana cost X generic mana green mana green mana green mana ) and choose X to be 3, the total cost is 3 generic mana green mana green mana green mana . If you tap two green creatures and two red creatures, you'll have to pay 1 generic mana green mana .

If a creature you control has a mana ability with tap in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.