Bracing, which refers to the internal reinforcements on a guitar’s top and back, serves two primary functions: it keeps the guitar from collapsing under string tension and it shapes the guitar’s sound. Top bracing is perhaps the single most important factor in determining what a guitar will sound like. The vast majority steel-string guitars’ tops are braced with an “X” pattern, which was invented by Martin in the 1920s, and most often made from spruce. Forming an X that intersects just behind the soundhole and with two or three additional transverse braces (often called “tone bars”) behind the bridge, X-bracing has excellent stability and produces a balanced voice.
Some guitar tops feature variations on standard X-bracing like scalloped or tapered bracing. Scalloped braces are scooped out between two high points at each end of the brace, lending the look of suspension bridge. This reduces the brace’s weight and allows the luthier to control where the brace is flexible and where it is stiff. Guitars with scalloped bracing are typically more responsive, have more bass, and are sometimes louder than guitars with “straight” braces. Tapered braces are tallest at the center of the “X” and thin out evenly toward the guitar’s sides, often resulting in a very balanced sound.
Excerpted From: Understanding Your Guitar