From Academic Kids
An acoustic guitar is a modern form of guitar descended from the Classical guitar, but generally strung with steel strings for a brighter, louder sound. Much heavier construction is required to withstand the added tension. Sometimes the term steel-stringed guitar or folk guitar is used to differentiate from the classical guitar. The term "acoustic guitar" is a retronym, since before the invention of the electric instrument, "guitar" meant only the non-amplified variety.
The acoustic guitar is often played with one or more plectra, or guitar picks, small plates of hard plastic or other material used to strike and strum the strings. Most players use a flat pick held between the thumb and index finger. Some players use fingerpicks, picks worn on the thumb and three fingers. Fingerstyle acoustic guitar players use fingerpicks or use their bare fingers and fingernails to strum and strike individual notes.
There are many different variations on the construction of and materials used in acoustic guitars. More expensive guitars feature solid wood tops (usually Sitka spruce), sides and backs (often Indian rosewood, maple, or mahogany). Lower-priced guitars can combine solid tops with laminated backs and/or sides. Entry-level guitars are usually made entirely of laminated wood. Necks and fingerboards are usually made of denser, stiffer woods such as mahogany, rosewood, cedar and ebony. The various combinations of the different woods and their quality is the primary factor in the timbre or "tone" of the guitar. Due to decreasing availability and raising prices of the premium-quality woods, many manufacturers have begun experimenting with alternate species of woods or more commonly available variations on the standard species. Some have also begun using non-wood materials, such as plastic or graphite. Most luthiers and experienced players agree that a solid top is the most important factor in the tone of the guitar, and that solid backs and sides can also contribute to a pleasant sound, although laminated sides and backs are acceptable alternatives. Some jazz players prefer laminated backs and sides. (Please see Guitar for more details on the construction of acoustic guitars.)
There are several prominent American makers of acoustic guitars. Martin, Guild, Taylor, and Gibson are known for both the quality and price of their instruments. The Selmer-Maccaferri Guitar is an unusual looking guitar played by Django Reinhardt style musicians, distinctive by its "D" hole or longitudional oval soundhole.
The standard tuning for an acoustic guitar is EADGBE (low to high), although many players, particularly fingerpickers, use alternate tunings, such as "Open G" (DGDGBD), "Open D" (DADF#AD), or "Drop D" (DADGBE).
One variation on the standard acoustic guitar is the 12 string guitar, which sports an additional doubling string for each of the traditional six strings. This guitar was made famous by artists such as Huddie Leadbetter (a.k.a. Leadbelly), Pete Seeger, and Leo Kottke.