Tuner Monsters

Tuner monsters are a subtype of Monster card in the TCG and HCCG. They are much more plentiful in the game now than in the early days of the game. They are the only subtype whose members can be color-coded differently depending on what other kind of Monster card it is. This is because there exist Tuner Normal Monsters, Tuner Effect Monsters, Tuner Ritual Monsters, and Tuner Synchro Monsters (technically there could also be Tuner Fusion Monsters, but so far none have been released in HCCG).

The purpose of Tuner monsters is to perform a Synchro Summon. Every Synchro monster requires at least 1 Tuner monster as part of its Synchro Material, which is listed on the description of the Synchro monster being summoned.

The first Tuner monsters in HCCG debuted in Collection of Memories (Beach Ball Catty is a Tuner monster released in Light Beginnings, but this was errata'd in after the card had seen play in HCCG). Despite this, Tuner monsters really started permeating the metagame after the release of Underworld's Balance, since many splashable Tuners and Synchro monsters were created in that booster set. There are many Tuner monsters currently in the game, including some notable members of archetypes like Charmers, Tartaros, Tears, Oophagous, and Technicals.

To see a list of Tuner monsters in HCCG, see this page.