From Bulbapedia, the community-driven Pokémon encyclopedia.
The Sketch glitch is a glitch in Gold, Silver, and Crystal. A Pokémon using the move Sketch while transformed into another Pokémon will keep a Sketched move at the end of the battle, normally replacing Transform. A move can then be passed down as an Egg Move through breeding with a female Pokémon if the transforming Pokémon is Ditto or male.
In Generation II, it was commonly exploited with Ditto by using a Trainer House Trainer who has a Smeargle acquired using Mystery Gift. The most popular method was to face this Trainer in the Trainer House and have the user's Pokémon Transform into a Smeargle with at least one move other than Sketch. The user's Pokémon could then copy the other move via Sketch or if necessary defeat the Smeargle and Sketch another attack.
The Metronome glitch is an extension of the Sketch glitch. It was often exploited by using Pokémon who learn the move Metronome in Generation I, either naturally or from TM35 which could then be passed over to Generation II. For instance Gengar, a Pokémon which cannot naturally learn Transform, learns Metronome via TM35 in Generation I. Through Metronome it has a slim chance of using Transform, giving it the chance to copy a move via Sketch which replaces Metronome with this move at the end of the battle.
Combination with other glitches
Glitches with the potential of changing the species of another Pokémon such as the Pokémon merge glitch can also be used as part of extensions of the Sketch glitch in order to teach Pokémon Transform or Metronome to exploit the Sketch glitch.
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