A double stop, in music terminology, is the act of playing two notes simultaneously on a stringed instrument, for example a violin, a viola, a cello or a guitar. In performing a double stop, two separate strings are depressed (stopped) by the fingers, and bowed or plucked simultaneously. Double stops are exclusive to stringed instruments; for example, double-stops cannot be performed on wind instruments.
Likewise, the triple stop (three strings) and quadruple stop (four strings). Collectively, double, triple and quadruple stopping are called multiple stopping.
On instruments having a curved bridge, it is difficult to bow more than two strings simultaneously. The style of bow used until around the end of the 18th century, particularly in Germany, had the wood curved outwards (away from the hair), which made it somewhat easier to play three notes at the same time. However, most treatises written around the time make it clear that composers did not expect three notes to be played at once, even though the notes may be written in a way as to suggest this. Playing four notes at once is almost impossible, even with older bows. The normal way of playing three or four note chords is to briefly sound the lower notes and allow them to ring while the bow plays the upper notes (a broken chord). This gives the illusion of a true triple or quadruple stop. In forte, however, even with a modern violin and bow it is quite possible to play three notes at once, especially when played a little more towards the fingerboard. Obviously, with this technique, a little more pressure than usual is needed on the bow, so this cannot be practised in softer passages. Of course, great skill is needed for the violinist to keep a beautiful sound. This technique is mainly used in music with great force, like Russian music.
A 20th century invention by Emil Telmányi called the Bach bow makes use of a system of levers to temporarily slacken the bow hair and allow sustained three or four note chords; this design has no historical precedent and is less authentic than an ordinary modern bow for playing baroque music.
In addition to the style of bow, the curvature of the bridge is an important factor in the ease of multiple stopping. On most classical instruments, the bridge is curved enough to make it difficult to play three strings at once, but on some violins the bridge is shaved down until almost flat, making it far easier to triple stop, as well as to alternate double stopping on different pairs of strings (D-A to A-E for example). The compensating disadvantage is that more skill is needed to avoid playing a double stop when none is called for.
When playing rock guitar styles, the most common form of double stop is a perfect fifth (or its inversion, a perfect fourth), which is one type of power chord. These double stops are often found in distorted, over-driven guitar parts where the tempo is such that the full chord cannot be played or the sound would be too muddied, though they are also common in quieter rock and blues music. The bass technique kown as tapping uses double stopping often, but is not popular because it doesn't sound as "bass" as it does guitar.
Double stops are also used in tuned percussion, such as on the vibraphone or marimba, and more rarely, timpani. A percussion double stop simply consists of striking both bars or timpani with two separate mallets.
(Courtesy of Wikipedia)
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