They theorize that a decrease in the content of hemicellulose, crystallization of cellulose, and changes to lignin over time all result in its wood gaining better resonating properties.

A common non-traditional wood gaining popularity is sapele, which is tonally similar to mahogany but slightly lighter in color and possessing a deep grain structure that is visually appealing.

Carbon-fiber and phenolic composite materials have become desirable for building necks, and some high-end luthiers produce all-carbon-fiber guitars.

One innovation is a metal bar called a truss rod, which is incorporated into the neck to strengthen it and provide adjustable counter-tension to the stress of the strings.

A critical structural and tonal component of an acoustic guitar is the bracing, a systems of struts glued to the inside of the back and top.

Innovations in bracing design have emerged, notably the A-brace developed by British luthier Roger Bucknall of Fylde Guitars.

Most luthiers and experienced players agree that a good solid top (as opposed to laminated or plywood) is the most important factor in the tone of the guitar.

Until the 1960s, the predominant forms of music played on the flat-top, steel-string guitar remained relatively stable and included acoustic blues, country, bluegrass, folk, and several genres of rock.

The concept of playing solo steel-string guitar in a concert setting was introduced in the early 1960s by such performers as Davey Graham and John Fahey, who used country blues fingerpicking techniques to compose original compositions with structures somewhat like European classical music.

Fahey contemporary Robbie Basho added elements of Indian classical music and Leo Kottke used a Faheyesque approach to make the first solo steel-string guitar "hit" record.

[citation needed] Steel-string guitars are also important in the world of flatpicking, as utilized by such artists as Clarence White, Tony Rice, Bryan Sutton, Doc Watson and David Grier.