[gitnoob]

http://www.guitar-maker.com/Pages/histSSG.html

German immigrants to America started making guitars with an X-brace under the soundboard. Whether or not Martin developed this himself is not known, but he certainly made it his own, by the 1850's most of his guitars were built this way. The advantage of this arrangement was probably mainly commercial at this stage, as the X-brace used less wood than Torres' fan-struts. The main virtue of the X-brace lay unsuspected by makers and players alike for the next fifty years.

One of the big disadvantages of the early guitar was its lack of volume. Torres made great improvements with the wider bodies and fan-strutting of his guitars, but American guitarists wanted an instrument which could hold its own when played alongside much louder banjos, mandolins and fiddles at barn dances and the like. They got what they wanted around 1900, when steel strings came out. But steel strings exert more than double the tension on a guitar soundboard that gut strings do. A slight strengthening of the X-brace was all that was needed to cope with the extra tension, and by the 1920's it had become an industry standard for the steel-string guitar.