A mandolin is a stringed musical instrument. Mandolins have 8 strings, in 4 pairs (or courses). Each pair of strings is tuned in unison, and are a fifth apart from adjacent pairs, giving an identical tuning to a violin (G-D-A-E low-to-high). Unlike a violin, the fingerboard of a mandolin is fretted and it is typically played with a flat pick (a plectrum).

Like the guitar, the mandolin is a poorly sustaining instrument --- a note cannot be maintained for an arbitrary time as with a violin. Its higher pitch makes this problem more severe than with the guitar, and as a result, use of tremolo (rapid picking on a single note) is sometimes used to emulate a sustained note.

Mandolins come in a few forms. The Neapolitan style, known as a round or bowl-back, has a vaulted back made of a number of strips of wood in a bowl formation, similar to a lute and usually a two-plane uncarved (flat) top. The Portuguese, a flat-back style is derived from the cittern. Another form has a banjo-style body (the body is about half the size of a banjo.)

Carved and round backed mandolins

In the early twentieth century, a new mandolin-style with carved top and back construction, as employed in violin family instruments, began to supplant the European-style instruments, especially in the United States. This new style is credited to mandolins designed and built by Orville Gibson who founded the Gibson company in 1902. Gibson mandolins evolved into two families: the F-style, which has a scroll near the neck and two points on the right side; and the A-style, which is pear shaped and has no points. These styles generally have either two f-shaped soundholes like a violin or an oval sound hole directly under the strings. Naturally, there is much variation among makers, and different styles exist as well, but these are the most common. The F-hole, F-style mandolins are considered the most typical and traditional for bluegrass, while A-style with oval hole is generally more appropriate for Irish, folk, or classical music.

Numerous modern mandolin makers build instruments that are largely replicas of the Gibson F-5 Artist models built in the early 1920s by Gibson acoustician Lloyd Loar. Original Loar-signed instruments are sought-after and extremely valuable.

Larger versions of the mandolin are the mandola (a fifth below the mandolin, as the viola is below the violin), the octave mandolin (an octave below the mandolin), and the mandocello, which is tuned an octave plus a fifth below the mandolin (like a cello). All of these have 8 strings tuned in unison.

Mandolins have a long history and much early music was written for them. However they are now mainly heard in country, Old-time music, bluegrass and folk music.