Anatomy Of A Classical Guitar
Parts of the guitar
- 1 Headstock
- 2 Nut
- 3 Machine heads (or pegheads, tuning keys, tuning machines, tuners)
- 4 Fretwires
- 5 Truss rod (not shown)
- 7 Neck and 20 fretboard
- 8 Heel
- 9 Body
- 12 Bridge
- 14 Bottom deck
- 15 Face (top deck)
- 16 Body sides
- 17 Sound hole, with Rosette inlay
- 18 Strings
- 19 Bridge saddle (Bridge nut)
- 20 The Fretboard
The fretboard (also called the fingerboard) is a piece of wood embedded with metal fretwires that constitutes the top of the neck. It is flat or slightly curved. The curvature of the fretboard is measured by the fretboard radius, which is the radius of a hypothetical circle of which the fretboard’s surface constitutes a segment. The smaller the fretboard radius, the more noticeably curved the fretboard is. Pinching a string against the fretboard effectively shortens the vibrating length of the string, producing a higher tone (a string, unfingered, will vibrate from the saddle to the nut; once fingered, it will vibrate only along the distance between the saddle and the fretwire directly before the finger). Fretboards are most commonly made of ebony, but may also be made of rosewood or of phenolic composite (“micarta”).
Frets are the metal strips (usually nickel alloy or stainless steel) embedded along the fingerboard and placed at points that divide the length of string mathematically. The strings’ vibrating length is determined when the strings are pressed down behind the frets. Each fret produces a different pitch and each pitch spaced a half-step apart on the 12 tone scale. The ratio of the widths of two consecutive frets is the twelfth root of two , whose numeric value is about 1.059463. The twelfth fret divides the string in two exact halves and the 24th fret (if present) divides the string in half yet again. Every twelve frets represents one octave. This arrangement of frets results in equal tempered tuning. For more on fret spacing, see the Strings and Tuning section.
Frets are placed at fractions of the length of a string (the string midpoint is at the 12th fret; one-third the length of the string reaches from the nut to the 7th fret, the 7th fret to the 19th, and the 19th to the saddle; one-quarter reaches from nut to fifth to twelfth to twenty-fourth to saddle). This feature is helpful when playing harmonics.
Frets are usually the first permanent part to wear out on a heavily played guitar. They can be re-shaped to a certain extent and can be replaced as needed. Frets are available in several different gauges, depending on the type of guitar and the player’s requirements.
The truss rod is an adjustable metal rod that runs along the inside of the neck, adjusted by a hex key or an allen-wrench bolt usually located either at the headstock (under a cover) or just inside the body of the guitar, underneath the fretboard (accessible through the sound hole). Most classical guitars do not have truss rods, as the nylon strings do not put enough tension on the neck for one to be needed. The truss rod counteracts the immense amount of tension the strings place on the neck, bringing the neck back to a straighter position. The truss rod can be adjusted to compensate for changes in the neck wood due to changes in humidity or to compensate for changes in the tension of strings. Tightening the rod will curve the neck back and loosening it will return it forward. Adjusting the truss rod affects the intonation of a guitar as well as affecting the action (the height of the strings from the fingerboard). Some truss rod systems, called “double action” truss systems, will tighten both ways, allowing the neck to be pushed both forward and backward (most truss rods can only be loosened so much, beyond which the bolt will just come loose and the neck will no longer be pulled forward).
A classical guitar’s frets, fretboard, tuners, headstock, and truss rod, all attached to a long wooden extension, collectively constitute its neck. The wood used to make the fretboard will usually differ from the wood in the rest of the neck. The bending stress on the neck is considerable, particularly when heavier gauge strings are used (see Strings and tuning), and the ability of the neck to resist bending (see Truss rod) is important to the guitar’s ability to hold a constant pitch during tuning or when strings are fretted. The rigidity of the neck with respect to the body of the guitar is one determinant of a good instrument versus a poor one. The shape of the back of the neck can also vary, from a gentle “C” curve to a more pronounced “V” curve.
Neck joint or ‘heel’
This is the point at which the neck meets the body of the guitar. In the traditional Spanish neck joint the neck and block are one piece with the sides inserted into slots cut in the block. Other necks are built separately and joined to the body either with a dovetail joint, mortise or flush joint. These joints are usually glued and can be reinforced with mechanical fasteners. Recently many manufacturers use bolt on fasteners. Bolt on neck joints were once associated only with less expensive instruments but now some top manufacturers and hand builders are using variations of this method. Some people believed that the Spanish style one piece neck/block and glued dovetail necks have better sustain, but testing has failed to confirm this. While most traditional Spanish style builders use the one piece neck/heel block, Fleta a prominent Spanish builder used a dovetail joint due to the influence of his early training in violin making. One reason for the introduction of the mechanical joints was to make it easier to repair necks. This is more of a problem with steel string guitars than with nylon strings which have about half the string tension. This is why nylon string guitars often don’t include a truss rod either.
The body of the instrument is a major determinant of the overall sound variety for acoustic guitars. The guitar top, or soundboard, is a finely crafted and engineered element often made of spruce, red cedar or mahogany. This thin (often 2 or 3 mm thick) piece of wood, strengthened by different types of internal bracing, is considered to be the most prominent factor in determining the sound quality of a guitar. The majority of the sound is caused by vibration of the guitar top as the energy of the vibrating strings is transferred to it. Different patterns of wood bracing have been used through the years by luthiers (Torres, Hauser, Ramírez, Fleta, and C.F. Martin being among the most influential designers of their times); to not only strengthen the top against collapsing under the tremendous stress exerted by the tensioned strings, but also to affect the resonation of the top. The back and sides are made out of a variety of woods such as mahogany, Indian rosewood and highly regarded Brazilian rosewood (Dalbergia nigra). Each one is chosen for its aesthetic effect and structural strength, and such choice can also play a significant role in determining the instrument’s timbre. These are also strengthened with internal bracing, and decorated with inlays and purfling.
The body of a classical guitar is a resonating chamber which projects the vibrations of the body through a sound hole, allowing the acoustic guitar to be heard without amplification. The sound hole is normally a round hole in the top of the guitar (under the strings), though some may have different placement, shapes or multiple holes.
An instrument’s maximum volume is determined by how much air it can move.
Binding, purfling and kerfing
The top, back and rim of a classical guitar body are very thin (1-2 mm), so a flexible piece of wood called kerfing (because it is often scored, or kerfed to allow it to bend with the shape of the rim) is glued into the corners where the rim meets the top and back. This interior reinforcement provides 5 to 20 mm of solid gluing area for these corner joints.
During final construction, a small section of the outside corners is carved or routed out and then filled with binding material on the outside corners and decorative strips of material next to the binding, which are called purfling. This binding serves to seal off the endgrain of the top and back. Binding and purfling materials are generally made of either wood or high quality plastic materials.
The main purpose of the bridge on a classical guitar is to transfer the vibration from the strings to the soundboard, which vibrates the air inside of the guitar, thereby amplifying the sound produced by the strings. The bridge holds the strings in place on the body. Also, the position of the saddle, usually a strip of bone or plastic across the bridge upon which the strings rest, determines the distance to the nut (at the top of the fingerboard). This distance defines the positions of the harmonic nodes for the strings over the fretboard, and is the basis of intonation. Intonation refers to the property that the actual frequency of each string at each fret matches what those frequencies should be according to music theory. Because of the physical limitations of fretted instruments, intonation is at best approximate; thus, the guitar’s intonation is said to be tempered. The twelfth, or octave, fret resides directly under the first harmonic node (half-length of the string), and in the tempered fretboard, the ratio of distances between consecutive frets is approximately 1.06 (see “Frets” above).
The modern full size classical guitar has a scale size of around 650 mm (25.6 inches), with an overall instrument length of 965-1016 mm (38-40 inches). The scale size has remained consistently between 640-650 mm (25.2- 25.6 inches) since 650 mm was chosen by the originator of the instrument, Antonio de Torres. This length was probably chosen as twice the length of a violin string. As the guitar is tuned to one octave below that of the violin, the same size gut could be used for the 1st strings of both instruments.
Smaller scale instruments are produced to assist children in learning the instrument as the smaller scale leads to the frets being closer together making it easier for smaller hands. The scale size for the smaller guitars is usually in the range 484-578 mm (19-22.5 inches) with an instrument length of 785-915 mm (31-36 inches). Full size instruments are sometimes referred to as 4/4, while the smaller sizes are 3/4, 1/2 or 1/4.
Scale size table
These sizes are not absolute as luthiers may choose variations around these nominal sizes.
- 4/4 650 mm (25.6 inches)
- 3/4 578 mm (22.75 inches)
- 1/2 522 mm (20.5 inches)
- 1/4 484 mm (19 inches)