A mandolin will usually have a hollow wooden body with a tailpiece that holds one end of the strings, a floating bridge, a neck with a flat (or slight radius) fretted fingerboard, a nut, and mechanical tuning machines to accommodate metal strings. There are, of course many tunings you can use on a mandolin, though the most common tuning is GDAE, which is the same as the common violin tuning.
As with any plucked instrument, the mandolin notes decay to silence rather than sound out continuously as with a bowed note on a violin. It's small size and high pitch make mandolin notes decay faster than larger stringed instruments like guitar, which encourages the use of tremolo (rapid picking of one or more pairs of strings) to create sustained notes or chords. The mandolin's paired strings facilitate this technique: the pick strikes each of a pair of strings alternately which provides a more full and continuous sound than a single string would.
Various design variations and amplification techniques have been used to make mandolins compatible in volume with louder instruments and orchestras. Hybridization with the louder banjo creates the mandolin-banjo, and resonators have been used, most notably by Dobro and the National String Instrument Corporation. Some musicians play electric mandolins through amplifiers.
Early mandolins had six double courses of gut strings, tuned similarly to lutes and were plucked with the fingertips. Modern mandolins, which originated in Naples, Italy in the late 18th century, tend to have four double courses (four pairs) of metal strings which are plucked with a pick.
Many variants of the mandolin have existed. These include Milanese, Lombard, Brescian and other six-course types, as well as four-string (one string per course), twelve-string (three strings per course), and sixteen-string (four strings per course).
Mandolins can come in several different forms. The bowl-back/round-back a.k.a. "Neapolitan style" is made similar to a lute, with a "bowl back", using strips of wood to form the bowl and usually has a canted, two-plane, uncarved top. There is another form that has a banjo-style body (the banjo mandolin).
Another style is the archtop, which came into existence towards the end of the 19th century. They have a carved top and back construction similar/inspired by the violin style of instrument. The archtop style is credited to Orville Gibson. Gibson mandolins evolved into two basic styles: the Florentine or F-style, which has a decorative scroll near the neck, two points on the lower body, and usually a scroll carved into the headstock; and the A-style, which is pear shaped, has no points, and usually has a simpler headstock. These styles will generally have either two F-shaped soundholes like a violin (F-5 and A-5), or an oval sound hole (F-4 and A-4 and lower models) directly under the strings.
A lot of variation exists between makers working from these archetypes and other variants have become increasingly common. Often in the United States, Gibson F-hole F-5 mandolins and mandolins influenced by that design are strongly associated with bluegrass, while the A-style is associated other types of music, although it too is most often used for and associated with bluegrass.
The F-5's more complicated woodwork also translates into a more expensive instrument. In Ireland and Britain, flat top and back instruments are often used. Internal bracing to support the top in the F-style mandolins is usually achieved with parallel tone bars, similar to the bass bar on a violin. Some makers instead employ "x-bracing," which is two tone bars mortised together to form an X. Some luthiers now using a "modified x-bracing" that incorporates both a tone bar and x-bracing.
Many modern mandolin makers build their instruments to replicate the Gibson F-5 Artist models that were built early in the 1920s.
Also related to the mandolin are the mandola, bouzouki, mando-bass, mandocello, and octave mandolin.