A model of guitar made of a body and two separate necks with two separate fretboards. A standard guitar is made of a body and only one neck.
Most of the top guitar-making companies have produced their own doubleneck guitars.
The purpose of a doubleneck guitar, you ask? Many doubleneck guitars, like the one owned by Zakk Wylde incorporate the twelve string guitar and the standard six-string, one neck for each. Some may have one neck detuned, to achieve lower notes, chords, etc.
A unique guitar invented by Michael Bianco and built by Bob Mason, the T14 is basically made of two seven string guitars. Made of two fretboards on one piece of wood, this guitar is mostly used for fingertapping, and can be compared to the Chapman stick.