Slide guitar is a particular method for playing the guitar. Instead of altering the pitch of the strings in the normal manner, by pressing the strings against the fretboard, a slide is used. The slide is a tube of some hard material. Commonly these were made from glass (or a bottleneck, which gave rise to the description bottleneck guitar, although metals (normally steel or brass) are also used. The slide is pressed against the strings (but not the fret board), producing a sustained tone which can be continuously varied.
In this style the guitar is often tuned to an open chord, such as open D or open G, allowing the player to change chords simply by moving the slide with one hand, without holding down any strings with the other. This method is the easiest way of playing chords in slide guitar, and is most common; but many experienced guitarists use other tunings as well.
Some specialised makes of guitar are commonly used to play slide. These include resonator guitars, commonly also known as dobro's after the company which first made them. These guitars are made with steel bodies. Some have a square neck rather than the more normal curved neck and are designed to be played across the knee.