Circle of Protection: White

The original sideboard hoser, and the template that every color-hate enchantment since has had to answer to. The design is unusually honest about what it costs to hate out a color: two mana to deploy, then a generic mana per damage event, with the player naming which source to blank. That repeated, per-event payment is what keeps the lock from running away; against a board of multiple white attackers, the costs stack up fast and the Circle never becomes a single-card answer to everything. The "of your choice" phrasing is the famous quirk: because the ability does not target, it sails past shroud, hexproof, and even protection, blanking sources that a targeted prevention spell could never touch, while still asking you to pay for each one individually. The cycle (one Circle for each color, printed in the original starter sets) was Wizards teaching new players what a sideboard was for, back when the sideboard concept itself was still being formalized in tournament rules. The Circles were the training wheels: a clean, readable answer to "the other deck is mono-something." Later color hate got narrower and cheaper, but the Circle's shape (a repeatable, mana-gated, non-targeting prevention effect) is the ancestor of all of it. The prevention-damage design space has largely been retired, which makes the Circles a museum piece: the clearest surviving record of a time when Wizards expected players to build sideboards around specific colors rather than specific strategies.