A flat top is simply a guitar whose top is flat. The thickness of the top determines the tone of the instrument. The guitar top, or soundboard, is a finely crafted and engineered element often made of spruce, cedar, or mahogany. This thin (often 2 or 3mm. thick) piece of wood, strengthened by different types of internal bracing, is considered to be the most prominent factor in determining the sound quality of a guitar. The majority of the sound is caused by vibration of the guitar top as the energy of the vibrating strings is transferred to it. Different patterns of wood bracing have been used through the years by luthiers; to not only strengthen the top against collapsing under the tremendous stress exerted by the tensioned strings, but also to affect the resonation of the top.
The term “flat top” usually refers to the acoustic steel string guitar, such as a Martin D-28, but is also applied to electric guitars. An example would be the Fender Telecaster or perhaps a Gibson SG. The body size of an acoustic flat top is usually significantly larger than a classical guitar and it has a narrower, reinforced neck and stronger structural design, to sustain the extra tension of steel strings which produce a louder and brighter tone. The acoustic guitar is a staple in folk, old-time music, and blues.