The Guitar Grip is required to play the game and comes as part of a bundle that can be purchased for each installment. The Grip is designed for the DS Lite, but features a small adapter that can be removed for use in the older Nintendo DS models. Four fret buttons are located on the side of the unit near the cartridge slot. This is one less than the normal five frets, included an orange-colored one, managed by other Guitar Hero controllers. A wriststrap is attached to the underside to provide support while playing. The player holds the unit in a vertical book orientation (similar to Ninja Gaiden: Dragon Sword or Hotel Dusk: Room 215), and uses a special guitar pick-shaped stylus to strum on the touchscreen of the DS with his or her free hand.
Source: Wikipedia, "Guitar Hero: On Tour series", available under the CC-BY-SA License.