http://en.wikipedia.org/wiki/Archtop_guitar
An archtop guitar is a steel-stringed acoustic or semi-acoustic guitar with a full body and a distinctive arched top, whose sound is particularly popular with blues and jazz players.
Typically, an archtop has:
* An arched top and back, not flat
* Moveable adjustable bridge
* F-holes similar to members of the violin family.
* Humbucker pickups.
* Rear mounted tailpiece
Although archtop normally refers to a hollow-bodied instrument, some makers of solid-bodied guitars with carved bellies also refer to these as archtop to distinguish these from flat top guitars. For example, Gibson refer to the standard Gibson Les Paul as an arch top to distinguish it from flat top models such as the Les Paul Junior and Melody Maker.
A continuum exists from these solid body, purely electric instruments to purely acoustic instruments similar to the original Orville Gibson design, including:
* Solid body instruments, such as the Les Paul standard, with a carved but non-sounding belly.
* Instruments with a solid core but hollow bouts, such as the Gibson ES-335. In these, the bridge is fixed to a solid block of wood rather than to a sounding board, and the belly vibration is minimised much as in a solid body instrument.
* Thin-bodied semi-acoustic instruments, such as the Epiphone Casino. These possess both a sounding board and sound box, but the function of these is purely to modify the sound transmitted to the pickups. Such guitars are still intended purely as electric instruments, and while they do make some sound when the pickups are not used, the tone is weak and not normally considered musically useful.
* Full hollowbody semi-acoustic instruments, such as the Gibson ES-175; these have a full-size sound box, but are still intended to be played through an amplifier.
* Prototypical archtops, such as the Gibson L-5, although most often fitted with one or more pickups and normally amplified if pickups are present, also retain a full-size body and a powerful acoustic tone suitable both for chords and for melody work.
All of these types may be loosely described as archtop, but only the last possesses the characteristics most often associated with the type.