The first step in making a guitar is selecting the woods. The top of the body, or the sound board, is often made of spruce or cedar because of their superior tonal qualities. The back and sides of the guitar are usually made of mahogany, alder, ash or walnut. Necks are often made of maple, mahogany, koa or rosewood. Although they have an effect on guitar tone, woods in the body and neck have less of an effect than the wood in the sound board.
Acoustic guitars are manufactured by cutting the shape of the top and back in the traditional figure-eight guitar shape. The sound hole or holes are cut into the top, or sound board. The sides are then steamed to soften the wood. The wood is then dried in a mold so it retains the traditional curves of the guitar. The top, bottom and sides are then glued to an internal bracing system to give the guitar strength.
Necks are either carved by hand or, more commonly, created with a computer-controlled cutter. Once the shape of the neck has been created, a fingerboard is glued on and shaped. Frets are inserted into the fingerboard. Most acoustic guitars then need to have a headstock attached for tuners. Once the headstock is joined and glued in place, the neck is ready to be finished.
Steel-string guitar necks contain a component usually not found in nylon-string guitars. The tensions created by steel strings often require the placement of a metal truss rod in a channel under the fingerboard.
The finish on a guitar has an effect on the resonance of the underlying woods. Different finishes allow for more or less resonance. Traditional finishes include nitrocellulose polyurethane. Nitrocellulose is applied in repeated thin layers that move more freely and allow for more resonance from a guitar. Many modern guitars use multiple layers of polyurethane. Although not as flammable as nitrocellulose, polyurethane doesn't allow the wood to resonate as freely. An advantage of polyurethane over nitrocellulose is its durability.
Once the guitar is finished, assembly involves joining the neck to the body. This is sometimes done with bolts, sometimes with glue. Certain guitar makers will use both bolts and glue. When the guitar neck and body are joined, the guitar's nut, tuners and bridge are installed.
Once the basic guitar is finished, strings are attached and the guitar is set up. Set-up involves filing the grooves in the nuts and filing the bridge to position the strings at the desired height. In steel-string guitars, the truss rod is adjusted to keep the fretboard flat.