Tuner (monster)[Notes 2]
Tuner monsters (Japanese: チューナー Chūnā, "Tuner") are monsters with "Tuner" listed on the Type/Ability line. They are required for the Synchro Summon of a Synchro Monster. Normal, Fusion, Effect, Pendulum, and Synchro Monsters themselves can all be Tuner monsters, although some card effects can treat non-Tuner monsters as Tuners. Flip and Union monsters and Special Summon Monsters can also be Tuner monsters.
To perform a Synchro Summon, the player sends monsters they control (including a Tuner monster) to the Graveyard whose total Level is exactly equal to that of a Synchro Monster in their Extra Deck, then they Synchro Summon that Synchro Monster. Usually, Synchro Summons require exactly 1 Tuner monster, but some Synchro Monsters allow or require multiple Tuner monsters (referred to as Double Tuning).
Some Synchro Monsters require a specific Tuner monster to be used ("Sun Dragon Inti"), or a Tuner of a specific Type ("Trident Dragion"), Attribute ("Dragocytos Corrupted Nethersoul Dragon"), or archetype ("Blackwing Tamer - Obsidian Hawk Joe"), etc. Likewise, some Tuners can only be used as Synchro Material for a limited set of Synchro Monsters, such as "Brotherhood of the Fire Fist - Spirit", "Mystic Macrocarpa Seed", and "Scrap Soldier".
There are also Tuner Synchro Monsters, Synchro Monsters that are themselves Tuners. Due to their difficulty of use, they tend to have effects that make Synchro Summoning easier, such as by manipulating their own Level ("Phonon Pulse Dragon"), allowing its controller to Synchro Summon during the opponent's turn ("T.G. Wonder Magician"), or having effects that activate when used as a Synchro Material ("Puralis, the Purple Pyrotile"). In the anime, Tuner Synchro Monsters are notably required for Accel Synchro Monsters, such as "Shooting Star Dragon".
Tuner monsters are often low in Level, usually ranging from Level 1 to 3. A number of Level 4 Tuner monsters exist, but very frequently have negative effects or conditions that limit their use (due to Level 4 being by far the most common Level for monsters that can be readily Normal Summoned, this is presumably to make it more difficult to Summon Level 8 Synchro Monsters). The first Level 4 Normal Tuner monster, "Angel Trumpeter", was not released until 2016, eight years after the debut of Tuner and Synchro monsters. There are not many Level 5 or higher Tuner monsters.
Many Tuner monsters have effects that assist in performing a Synchro Summon, such as by Special Summoning another monster ("Deep Sea Diva", "Masked Chameleon"), Special Summoning themselves when a condition is met ("Lady of the Lake", "Fishborg Archer"), or managing the Level, Type, or Attribute of monsters ("Delta Flyer", "Genomix Fighter", and "Genex Ally Chemistrer", respectively). One monster, "Phantom King Hydride", can treat itself as a non-Tuner monster for a Synchro Summon.
As implied by the name, several Tuner monsters and their designs have ties to sound, like "Frequency Magician", "Phonon Pulse Dragon", and the "Resonator" monsters. Many Tuner support cards have references to music or wavelengths, like "Natural Tune", "Resonator Call", and "Tuning".
Tuner Pendulum Monsters usually have an effect that restricts how they can be used as a Synchro Material and/or causes them to banish themselves if used as a Synchro Material. This is because otherwise they could easily be Pendulum Summoned again the next turn, to be used as Synchro Material again.