What it does
Harmonize is a casting-cost discount mechanic found on certain instants and sorceries. When you cast a spell with harmonize, you may tap any number of untapped creatures you control to help pay for it, with each tapped creature reducing the cost based on its power (or sometimes a set amount, depending on the card's wording). Effectively, your board of creatures helps "channel" the spell.
The nuance players miss: tapping creatures for harmonize isn't an activated ability or a cost reduction you can respond to separately—it's part of casting the spell, declared during the cost-payment step. The creatures don't need haste or to have been under your control since your last turn, since you're tapping them rather than attacking. Summoning sickness only restricts attacking and tapping for abilities with the tap symbol, not this.
In Commander, harmonize rewards wide creature boards, letting token strategies or go-wide decks cast big spells cheaply while still keeping mana open for interaction.
Iconic cards with Harmonize
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