Flat-top or steel-string guitars are similar to the classical guitar, however, within the varied sizes of the steel-stringed guitar the body size is usually significantly larger than a classical guitar, and has a narrower, reinforced neck and stronger structural design. The robust X-bracing typical of the steel-string was developed in the 1840s by German-American luthiers, of whom Christian Friedrich "C. F." Martin is the best known. Originally used on gut-strung instruments, the strength of the system allowed the guitar to withstand the additional tension of steel strings when this fortunate combination arose in the early 20th century. The steel strings produce a brighter tone, and according to many players, a louder sound. The acoustic guitar is used in many kinds of music including folk, country, bluegrass, pop, jazz, and blues. Many variations are possible from the roughly classical-sized OO and Parlour to the large Dreadnought (the most commonly available type) and Jumbo. Ovation makes a modern variation, with a rounded back/side assembly molded from artificial materials.