Acoustic Guitar

Overview

An acoustic guitar has about twenty named parts, and every one of them exists to do one of three jobs: hold six strings at a precise tension, let a player shorten those strings against a fretted neck, and turn the strings' vibration into sound the room can hear. The Body Assembly is the resonator, the Neck Assembly is the fingerboard and the mechanism that keeps the instrument in tune, and the String Set is what the player actually touches. The two flat pieces of hardware that people notice last, the Pickguard and the Strap Button, protect the top and hang the instrument from the player.

This page describes a steel-string dreadnought, the most common acoustic guitar shape. A classical guitar has the same parts with nylon strings, a wider neck, a slotted headstock and a lighter build; an Electric Guitar replaces the hollow resonating body with a solid slab and magnetic pickups, and is covered on its own page.

The parts of a guitar, from the top down

Headstock. The Headstock Assembly is the flat, angled plate at the end of the neck that carries the six tuning machines. It is tilted back by roughly 15 degrees so the strings press down firmly on the nut instead of lifting out of their slots. Each tuning machine is a small worm-gear box: turning the button rotates a worm that turns a pinion on the string post, and the gear ratio (usually 14:1 or 18:1) is what lets a player make tiny pitch changes without the post slipping back.

Nut. The Nut is a small bar of bone, synthetic bone or hard plastic set at the joint between headstock and fretboard. Six shallow slots in it fix the spacing and the height of the strings at the end of their vibrating length. Along with the saddle at the other end, it defines the scale length, the 645 mm (25.4 in) between the two points where an open string is stopped.

Neck and truss rod. The Neck Wood Blank is a length of mahogany or maple carved to a rounded C or V profile behind the fretboard. Six steel strings at pitch pull on it with something like 75 kg of force, enough to bow it forward over time. Inside a channel under the fretboard sits the Truss Rod, a steel rod with an adjusting nut at one end: tightening it bends the neck back against the string pull, and setting exactly how much bow remains is the first step of every guitar setup. The Neck Heel is the thickened block where the neck meets the body, and the Dovetail Neck Joint, a tapered dovetail glued into the body's neck block, is what holds the two together.

Fretboard and frets. The Fretboard is a thin slab of ebony or rosewood glued to the face of the neck. Pressed into slots across it are twenty pieces of Fret Wire, nickel-silver wire with a T-shaped cross section; the crowned top is what the string is stopped against, and the barbed tang underneath grips the slot. Fret positions follow the twelfth-root-of-two rule, each fret 5.6 percent closer to the saddle than the one before it, so that every fret raises the pitch by exactly one semitone. Small Position Marker Inlay inlays at frets 3, 5, 7, 9, 12, 15 and 17 let the player find a position without counting.

Soundboard. The Spruce Soundboard, or top, is the single most important part for tone: a plate of solid Sitka spruce about 2.8 mm thick, bookmatched from two halves, that vibrates like a loudspeaker cone when the strings drive it through the bridge. Spruce is used because it is unusually stiff along the grain for its weight, so the top can be thin and responsive without collapsing under string tension. The sound hole in it is not where most of the sound comes out; it is the vent that lets the air inside the body act as a spring, which is what gives the instrument its low-end resonance around 100 Hz.

Bracing. Glued to the underside of the top is the Top Bracing, a lattice of spruce struts in the X pattern that Martin introduced in the 1850s. Two main braces cross just below the sound hole, with tone bars, finger braces and a bridge plate behind them. The bracing stops the thin top from folding under the strings while leaving it free to vibrate; scalloping, carving the braces down between their ends, removes mass where stiffness is not needed and is much of the difference between a boxy guitar and a rich one.

Back and sides. The Rosewood Back and the two pieces of Rosewood Side form the rest of the box. Rosewood, mahogany or maple is used, at about 2 mm thick; the sides are bent to shape with heat and moisture. The back is arched slightly and braced with ladder bars. Where the sides meet the top and back, strips of Kerfed Lining, notched wood lining, provide a wide glue surface; the Neck Block and End Block reinforce the two ends of the body where the neck attaches and the strap button is fitted.

Bridge and saddle. The Bridge Assembly is the ebony or rosewood block glued to the top below the sound hole. The strings anchor in six holes through it, held by tapered bridge pins, and pass over the saddle, a bone strip that is the second string stop. The saddle is set at a slight angle and its top is compensated, string by string, so that fretted notes stay in tune up the neck. Everything the strings do is transmitted into the top through this one part, which is why a loose or lifting bridge kills an acoustic guitar's sound.

Rosette, binding and finish. The Sound Hole Rosette around the sound hole and the Body Binding along the body edges are decorative, but both also seal end grain and protect edges that would otherwise chip. The Finish Lacquer, nitrocellulose lacquer on a traditional build or polyurethane on a production one, seals the wood against humidity changes; thin finishes are preferred because a thick coat damps the top.

Strings. The String Set on a steel-string guitar is two plain steel strings (high E and B) and four wound strings, a steel core wrapped in phosphor bronze or 80/20 bronze. Gauges are named by the high E: a "light" set runs .012 to .053 in. Heavier strings are louder and pull harder on the neck; lighter ones are easier to play and gentler on the top.

Pickguard and strap button. The Pickguard is a thin celluloid or plastic plate below the sound hole that takes the pick strikes that would otherwise wear through the finish and into the spruce. The Strap Button at the tail end is an end pin that doubles as an output jack on guitars fitted with an onboard pickup.

How the parts work together

Plucking a string sets it vibrating at a frequency fixed by its length, tension and mass. Almost none of that vibration reaches the air directly; a string is too thin to move much of it. Instead the string's motion rocks the saddle, the saddle drives the bridge, and the bridge pumps the whole soundboard up and down. The top's large area couples the vibration to the air, and the enclosed body, breathing through the sound hole, adds a resonance that reinforces the low notes. The back and sides reflect and colour what the top produces. Fretting a string shortens its vibrating length and raises its pitch; the truss rod, nut and saddle together keep the string just above the frets along the whole neck so every note rings clean.

Materials and construction

The tonewoods matter because wood's stiffness-to-weight ratio sets how a plate vibrates. Spruce for tops, rosewood or mahogany for backs and sides, mahogany for necks and ebony for fretboards and bridges is the classic recipe; laminated woods on budget instruments trade some resonance for stability. The metal parts are few: fret wire, tuning machines, the truss rod and the strings. Hide glue or aliphatic resin holds the box together, and a well-made acoustic can be taken apart at those glue joints for repair, which is why century-old guitars are still played.

Common failures

Most acoustic guitar problems come from humidity. A dry top sinks and cracks along the grain; a wet one swells and lifts the bridge. Neck relief drifts with the seasons and is corrected at the truss rod. Frets wear flat under the strings and are levelled or replaced. Saddles and nuts wear and are recut. Strings corrode and go dull and are the only part changed routinely, every few weeks to months depending on use.

Common questions

What are the parts of a guitar called?

From the top: headstock, tuning machines, nut, neck, fretboard, frets, position markers, heel, body (soundboard, back, sides), sound hole, rosette, bridge, saddle, bridge pins, pickguard, strap button, and the strings. Inside the body: bracing, kerfing, neck block and end block.

What are the dots on a guitar neck called?

Position markers or fret markers. They sit at frets 3, 5, 7, 9, 12, 15 and 17, with a double marker at the twelfth fret, which is the octave.

What is the difference between an acoustic and a classical guitar?

Both are hollow-bodied. A classical guitar uses nylon strings, has a wider and flatter neck that joins the body at the twelfth fret, a slotted headstock, a tie-block bridge instead of bridge pins, and fan bracing under a lighter top. A steel-string acoustic uses steel and bronze strings under far higher tension and is built heavier to take it.

What part of the guitar makes the sound?

The soundboard. The strings drive it through the saddle and bridge, and its vibration moves the air. The body cavity and sound hole add the low-frequency resonance; the back and sides shape the tone.

How many strings does a guitar have?

Six on a standard guitar, tuned E, A, D, G, B, E from lowest to highest. Twelve-string guitars pair each string with an octave or unison partner; bass guitars usually have four.