X-bracing refers to the pattern of the strips of wood used in luthierie to strengthen the top of an acoustic guitar. The term derives from the pattern of the main bracing on the inside surface of the guitar’s top. X-bracing was invented in the 1850s by C.F. Martin & Co to serve the needs of the modern steel-string guitar, as opposed to the traditional fan bracing of the classical guitar. This pattern comprises two main braces running in an “X” from the upper bout to the lower bout, with the “X” crossing somewhere between the soundhole and the bridge. There are several auxiliary braces as well. Bracing serves two seemingly disparate functions: Strengthen the top of the guitar in order to accommodate the 185lbs (approximately) of constant tension it must endure, while allowing the top to vibrate sufficiently to produce tone. A top thick enough to withstand the pressure could not vibrate properly. Interestingly enough, bracing also plays a major role in determining the tone of a guitar based on placement, pattern, and physical shape. In many ways, these elements affect the final tone of the instrument as much as the actual tonewood.