The Guitar Neck & Fretboard

On acoustic guitars the neck extends from the headstock to the heel joint at the top of the guitar body. The neck serves two functions. Mechanically, it supports the strings, but it also provides support for the main playing surface - the fretboard.

The neck and fretboard are perhaps the most critical part of the guitar as far as feel and playability are concerned. Our hands are very sensitive and to a guitarist, changes in neck dimensions of only a fraction of a millimetre can make a noticeable difference in the playing feel of a guitar. This level of accuracy is difficult to achieve and to control, when working in wood.

The neck - sustain and tone

There are many opinions about how the neck affects the tone of the guitar. Although the neck is always in contact with one end of the strings, which are the primary source of the sound of the guitar, it obviously isn’t directly involved in sound production. Any energy from the strings that vibrates the neck is really wasted and sustain is lost. Ideally the neck end of the strings shouldn’t vibrate at all and the neck shouldn’t adsorb any energy from the strings. In this case the neck wouldn’t affect or alter the sound of the guitar in any way and all the energy from the strings would go into driving the soundboard. Practically this isn’t possible, but a real neck (including the fretboard and frets) should be as stiff as possible and have a low internal damping factor. Modern materials such as carbon fibre, usually used as an insert, make it possible to make very stiff and strong necks, although currently only custom built and very high end production guitars feature this construction.

Some makers try to make the neck heavy, even going so far as adding a metal weight at the headstock and sometimes a counterbalance at the heel to increase the inertia and lower the resonant frequencies of the neck. Apparently, weighting the neck can increase volume and improve the treble response.

Neck woods – mahogany & maple

The wood used for the neck needs to provide good longitudinal strength and carve easily. The vast majority of acoustic guitar necks are made from mahogany. However the description ‘mahogany’ is pretty vague since there are two, completely different, species of wood commonly called mahogany. Each of these breaks down into numerous sub-species and to complicate matters even further, other completely unrelated woods such as, sapele (Entandophragma cylindricum), sipo or utile (Entandrophragma utile) and launa, are often sold as ‘mahogany’.

Big leafed or American mahogany (Swietenia macrophylla, often called South American, Brazilian or Peruvian mahogany, also caoba and acajou) from Central and South America is a true mahogany that is still available in commercial quantities and this is by far the most common wood used in acoustic guitar necks. Swietenia mahogani, once logged almost to extinction, is of the same group and is variously called West Indian mahogany, Spanish mahogany, or Cuban mahogany.

The second group of wood species referred to as mahogany are members of the Khaya group and depending on availability, are also used in neck construction. Since these species grow in Africa they are generally called African mahogany and include Khaya ivorensis, Khaya anthotheca and Khaya nyasica.

In general, the mahogany woods are light brown to reddish brown in colour and are almost always stained, for a darker, richer colour. Mahogany is lightweight but strong, carves well and has an attractive grain. However most mahogany is quite open grained, which can cause problems when trying to achieve a smooth finish and it can tear and split when cut against the grain.

Hard rock maple, rock maple, hard maple or sugar maple (Acer nigrum) is an excellent wood for guitar necks. It is most commonly used for electric guitar necks, although archtop guitars and resonator guitar necks are almost exclusively made from maple. It is very strong, carves well and has a tight grain. There are over 120 species of maple which are often grouped into ‘hard’ and ‘soft’ maple. Silverleaf maple (Acer saccharum) is one of the ‘soft’ maples, sometimes also used in guitar construction. Art & Lutherie and Simon & Patrick, both Canadian companies and part of the Godin group, use local Canadian silverleaf maple for their necks. Just to make things really confusing both acer nigrum and acer saccharum are sometimes referred to as ‘sugar’ maple because the sap of both trees can be used as maple syrup. Maple is generally heavier than mahogany, which is probably one reason why mahogany is preferred for acoustic guitar necks. Many players seem to prefer lightweight instruments and a heavy neck with a lightweight body may not balance well.

Nylon-strung guitar necks are sometimes made from Spanish cedar (Cedrela odorata and Cedrela mexicana) from Central and North America.

The best cut from a log for neck wood is where the growth rings of the wood lie at right angles to the fretboard. This aligns the natural structure of the wood so that it is at its stiffest against the pull of the strings. This is often referred to as 'quarter sawn' whereas in fact only some of the billets from a quarter sawn log exhibit grain at a true right angle to the wide face of the billet. Flat sawn logs are simply sliced into planks along the length of the log, while quarter sawn logs are first cut into quarters. Planks or billets are then cut from the alternate flat faces of each quarter, with the quarter rotated after each cut. Quartering and rotating a log in this way is a more labour intensive process than simple flat sawing, so quarter sawn timber is rarer and more expensive. However there will always be some timber from the flat sawn log that also has the the growth rings at right angles to its wide face.

Lengthwise laminated necks, made from a number of long strips of different woods glued together, enjoy a peculiar reputation. They are either found on low cost guitars, where they make economical use of timber that would otherwise be too small for a neck, or on very expensive guitars, where the aim is to produce both a stiff, stable neck and a visually decorative effect. Martin uses a material with the commercial name of Stratabond, a form of birch plywood, on their X series necks. Unlike the cross grain lamination found in standard plywood, in Stratabond the grain of each 2mm veneer, which are often stained in contrasting colours, runs parallel to the next, but does not line up perfectly, giving a subtle offset grain appearance.

Two views of laminated necks – one on a low cost Epiphone (which is an excellent guitar with very good stable neck) and the second on a custom built Olson guitar.

Two close ups of stained Stratabond, a material introduced quite recently by C.F.Martin on their X series necks.

Truss Rods

The truss rod is a metal rod, usually made of steel, inserted into a channel running along the length of the neck. Its purpose is to counteract the force of the strings, which tends to bend the neck forwards. Today there are two different types of threaded adjustable truss rod in common use, the single action, compression, or ‘vintage’ rod and the double (double action, double expansion) or bi-flex rod.
The single-action, compression type, rod is usually installed in a ‘concave’ channel cut into the neck. A filler stripe with a complementary curve is glued in to the channel over the rod. When the rod is tightened it tries to straighten and pushes up against the filler strip, making the neck bend backwards. The single rod can only counteract the force of the strings and straighten or bow the neck backwards. It does have the advantage that less wood has to be removed from the neck to fit it and the single rod has lower mass. The bi-flex or double action rods can bend the neck forwards or backwards, but add more mass and need a bigger channel carved in the neck. Broken single rods are often difficult to remove and replace. Double action rods aren’t anchored to the neck like the singles, so they can be easier to replace.

This cross sectioned neck shows a single action truss rod in situ, with the pale filler strip glued in above it (pic. by Frank Ford).

Nylon strung guitars aren’t usually fitted with truss rods because the tension of the strings is lower than on a steel strung and doesn’t bend the neck as much.

Truss rod adjustment

Wooden guitar necks may bend significantly forwards or backwards due to changes in temperature or humidity, requiring a small adjustment of the tension in the truss rod. A change of string gauge, or use of an altered tuning, may also require a truss rod adjustment. Normally only a quarter turn at most is needed, and although there is usually some resistance, the adjustment nut or screw should turn relatively easily. If a neck does not respond immediately it can be encouraged by bracing it across the knees and pushing firmly with a hand in the middle of the neck. Even with this encouragement most necks take time to settle after an adjustment. Exactly the right spanner or hex key should be used to make the adjustment so that controlled amounts of force can be applied without damaging the truss rod nut or socket. It’s important to know the type of truss rod and the required turning direction for the desired effect.

If a neck doesn’t respond and the adjustment won’t seem to turn without excessive force the rod may not be working properly. Removing the strings and slackening the rod until it is loose and then re-tensioning may help. Double action rods should have an obvious slack point between bending the neck forwards and backwards. With single action rods the adjustment nut can be removed and its threads lubricated with a small amount of Vaseline or paraffin wax (candle wax).

The heel joint

The ‘heel’ refers to the curved block of wood that is usually found on an acoustic guitar at the point where the neck joins the body. Traditional Spanish gut or nylon strung guitars don’t separate the neck and neck block at this point, the end of the neck is either carved in one piece or built up into a block that passes right inside the body and the sides are simply joined to this block. This approach can be used because high actions were and are regarded as quite acceptable on gut or nylon strung guitar, so fine control over the neck angle isn’t needed.

Prior to around 1929 the necks of steel strung guitars joined the body at the twelfth fret. Today the vast majority of guitars join, neck-to-body, at the fourteenth fret, providing a significant increase in the accessible note range of the guitar. Twelve-fret-to-the-body designs do offer an advantage, or at least a difference in tone, because the bridge is sited closer to the centre of the vibrating portion of the guitars top. This is also true of the 'classical' nylon strung guitar.

Steel strung guitars with a ‘traditional’ neck joint have a wood block inside the guitar and the neck is joined to this block with a dovetail joint. The dovetail is carved onto the inside face of the heel and secured to a matching cavity in the body block with glue, usually a hide glue is used. The end of the fretboard that extends over the body is also glued to the guitars top.

This method of joining the neck to the body is often referred to as a ‘set’ neck and although the neck angle can be controlled by trimming the dovetail, setting the initial neck angle is critical to playability and tone and requires great skill. Advocates of this method say that it provides the closest possible bond between the neck and the body and so produces the best tone and sustain. Unfortunately, over time, all acoustic guitars tend to hinge about the neck to body join and to suffer from a rising soundboard. Under the continual strain from the string tension, the action height gradually increases and the guitar becomes less and less playable. With a traditional set neck, correcting this problem, by re-setting the neck, is a highly skilled and expensive business.

In recent years manufacturers have started bolting necks onto the body. Initial setting and even precisely re-setting a bolt-on neck, using thin shims, is far easier than adjusting a glued, dovetail-jointed, neck. The use of this method has been pioneered and developed by Taylor Guitars. Although they had used a simpler, butt-joint, bolt-on system on their earlier production, in January 1999 Taylor introduced their NT or new technology neck joint. Taylor use state-of-the-art CNC (computer numerical control) cutting machines to cut precise mortises into the end of the body and into the top. Thin graduated wood shims can be laid into these mortises for precise adjustment of the neck angle. The neck heel and the fingerboard, supported by a solid extension of the neck, then fit very accurately into these mortises. Two bolts, one above the other, pass through two holes in the neck block on the body and screw into a threaded aluminium block set in to the heel on the neck. A third bolt passes through the top and holds the fingerboard neck extension firmly in place.

While the Taylor NT neck joint and other bolted neck joints, look from the outside, identical to a traditional neck joint, complete with the bulky external carved heel, there is another form of ‘bolt-on’ neck (usually the ‘bolts’ are long screws) that closely resembles the neck joint found on many electric guitars. In this joint the neck block inside the guitar is quite large and extends almost to the soundhole. A rectangular neck pocket is machined into the top of the guitar and the neck, which is square ended and extends the entire length of the fingerboard, drops into the pocket and is held in place by four long screws that pass through holes drilled in the back of the guitar. This joint, although not exactly elegant, has the advantage of simplicity and has no heel at all. It was originally used by Fender on their Kingman guitars and is currently used on some of the Tacoma range of instruments (ironically, Tacoma is now part of the Fender Musical Instrument Company group).

One solution to the neck set/reset problem is an adjustable neck. This idea first appeared in the 1800’s on the guitars of Johan Stauffer with a ‘hinged’ neck, adjusted by turning a square clock-key-style screw through a hole in the heel. This idea has recently been resurrected by Greg Smallman because he wanted to avoid his customers making action adjustments by filling down or replacing the saddle. Saddle and bridge height can have quite an effect on volume and tone, particularly on hand-tuned concert instruments.

Jeff Babicz has recently designed a range of guitars with a true ‘up-and-down’ neck adjustment that can be adjusted very easily, with no change in tuning, by turning a hex key inserted into a socket in the back of the guitar.

Guitars moulded out of plastics and composite materials can do away with the heel joint altogether because the neck, back and sides can be moulded in one piece. For example CA Guitars X Standard not only does away with the heel joint, but extends its sweeping cut-away behind the neck for unparalled access to the upper frets.

Neck profile, width and thickness

Guitarists at any level of ability will find a guitar with an accurate neck and a good setup easier to play. It also helps if the neck dimensions suite their hands. In an ideal world guitar neck dimensions should be tailored to the individual player’s hand. It’s not uncommon for famous players to have their favourite neck duplicated on a signature model, or for players to ask for necks based on their favourite features from several guitars, when commissioning custom built instruments. Perhaps in the future the leading guitar companies may be able to offer an off-the-peg neck fitting service to their general customers, based on hand measurements.

Older or vintage reissue instruments often feature a neck with a pronounced V profile. This neck shape provides a little extra strength and thickness down the centre spine, but compensates for that with shallow curved sides, so it isn’t too much of a handful. However this shape doesn’t appeal to everyone, so the vast majority of guitars made today settle for a generic semicircular profile.

All of the larger steel-strung acoustic guitar manufacturers today seem to be trying to please the habitual electric guitar player. So most acoustic necks are made with a fairly narrow nut width of around 43 to 44mm. These narrow necks require a very good setup and light gauge strings to play well. This is perhaps unfortunate since on an acoustic a wider neck is arguably better suited to fingerstyle playing and medium gauge strings provide better tone and dynamics.

It’s worth making sure that neck width allows enough room for clean fingering by comparing guitars with clearly different neck widths when shopping for a new acoustic. Larger makers, such as Martin and Taylor, offer a range of neck widths and shapes. Martin currently use four neck profiles – low oval, modified low oval, low profile and modified V and four neck widths at the nut 1 11/16 (42.86mm), 1 ¾ (44.45mm) 1 7/8 and 1 13/16ths (46.03mm). Taylor offer three neck widths - 1 11/16 (42.86mm), 1 ¾ (44.45mm) and 1 7/8 (47.62mm).
In Taylor’s case their bolt-on NT neck makes it possible to change necks for a different width. National Reso-Phonic single cone resonator guitars have a neck width of 46.35mm at the nut.

Compared to the steel strung, nylon strung guitar neck widths are commonly close to 50mm (2 inches). Although this for guitars with a flat fretboard and is making allowance for the thicker nylon treble strings it also indicates that complex and precise fingerings really need a wider ‘board.

Cross sections of the four neck profiles used by C.F. Martin.

The fretboard

The fingerboard or fretboard, a relatively thin slab of wood (usually no more than 6mm at its thickest) glued onto the front of the neck, is the main playing surface on the guitar. Metal frets, mounted in slots cut across the width of the ‘board, allow the guitarist to select the pitch of notes by pressing the string down behind a fret. Classical and flamenco, nylon strung guitars, always have a wide, flat, fretboard. Steel strung guitars often compensate for the extra force required to finger the strings by having a narrower neck with a cambered or radiused fretboard. The camber effectively bends a wider playing surface to fit a narrower neck, so reducing the strain on the player’s left hand. Camber radius on acoustic steel-strung guitars is typically between 12 to 16 inches, in general, the wider the neck, the flatter the radius.

Most guitar necks get wider towards the body, providing more finger area on the fetboard as the frets get closer together (the exceptions are the classical and the flamenco guitar with their very wide fretboards and some jazz archtops with narrow but almost constant width necks). When the fretboard is cambered on a tapered neck the camber should follow the contours of a cone rather than a cylinder. Although it’s easier to shape the fretboard into a cylindrical radius and cambered fretboards have often been shaped that way, in recent years the conical camber, known as a ‘compound radius’ has become popular. Although this is of more importance on electric guitars, where extreme string bending is often used, a compound radius on a tapered neck does provide a very comfortable playing feel.

Fretboard materials

Fretboards are traditionally made from some of the scarcer hardwoods, so guitar makers are starting to use synthetic materials, such as micarta or phenolic resin, even on guitars of otherwise very conservative design. Micarta is a trade name for composites made from layers of linen or paper bonded together by phenolic resin. Interestingly, Lignin is the natural phenolic polymer that gives wood its hardness and strength, so in effect, Micarta, made from natural plant fibre and phenolic resin, is a synthetic wood.

Indian rosewood also known as East Indian rosewood or Bombay rosewood (Dalbergia latifolia) is the most frequently used wood for fretboards because it has low internal damping (it is very resonant) its grain can look very attractive, it is hardwearing and, due to its natural oils, can be polished without an additional finish. It’s the dark heartwood that is used, the sapwood is a pale yellow-cream in colour. Unfortunately the quality of available rosewood is now so poor that new fretboards often look very dry and unappealing.

The other favourite fretboard wood is the dark heartwood from ebony trees. The sapwood is light grey and is known as white ebony. Various species of ebony grow in central Africa (Diospyrus crassiflora and Diospryus piscatoria) in Sri Lanka and India (Diospyrus ebenum, Diospyrus melanoxylon, Diospyrus tomentosa etc.). Like rosewood, ebony is also very hard wearing and has a low damping factor. Some players prefer ebony because its grain is much finer than that of rosewood. In its natural state even the heartwood ebony is not jet black and often exhibits grey and white streaks, so it is almost always stained to achieve a uniform deep black appearance, providing a nice contrast to inlays and fret markers.

Hard maple has on occasion been used for fretboards, most notably on the fretboards of resonator instruments made by the National company where it was always dyed black to resemble ebony.

The frets

Despite being only around 5mm thick the fretboard would help to stiffen the neck if it weren’t for the saw slots that hold the frets. Proper fret installation is critical to maintaining neck stiffness.

Over the years a number of different methods of fitting frets have evolved. Early instruments had tied-on frets made of gut. Today, metal frets are normally fitted to a fretboard by hammering, or pressing, the frets into the slots in the ‘board. Sometimes the frets are glued in to slots that are intentionally made for a slightly looser fit and sometimes a combination of force fit and glue is used. It’s very common to repair frets that are lifting at the ends with a drop of superglue and a clamp.

Most frets today are ‘T’ shaped with a rounded playing surface, or crown, on the top of the ‘T’. It’s the vertical leg of the ‘T’, called the fret tang, that is hammered in to the sawn slots and this has small teeth, or barbs, along its sides to help it hold in to the slot. Pre 1934, old Martin frets, known as bar frets, had no tang and were made from flat strip or bar fretwire.
Tang width and saw slot need to be correctly matched. As each fret is installed it tends to force the sides of the fret slot apart. The net result of installing all the frets is an overall back bow to the fretboard and neck. If the fret tang size and fret slots are correctly matched the pull from the strings will be enough to counteract this back bow and even pull the neck into a slight forward bow. This type of fret installation is known as compression fretting because first the frets tend to force the fret slots open, then the string pressure forces the sides of the fret slots closed, helping to hold the frets in place. With this method, if the saw slots are too wide the frets may be loose and neck stiffness can be affected.

With the glued method of fret installation the fret slots are almost a loose fit for the fret and it’s the glue, usually a hard epoxy glue, that is relied on to fill all the gaps around the fret and assure a stiff neck.

New frets are cut longer than needed and then trimmed and bevelled once they have been hammered, or pressed, into the fret slots. This often leaves an open end to the slot under each end of the fret. On un-bound necks these small gaps are filled before the neck finish is applied. Alternatively, neck binding is a more expensive method of concealing the slots. On a bound neck the edge of the ‘board is routed and the binding, of plastic, wood or bone, is glued on and trimmed after the fret slots have been cut. Each fret must then have the tang ground, or cut away, from each end, before it’s fitted, so that the remaining tang fits between the bindings. The ends of the fret crown then overlap the binding on either side.

Fretwire, which is available in a range of crown sizes, is almost always made of ‘German silver’ or ‘Nickel silver’ although stainless steel is also sometimes used. German/Nickel silver has nothing to do with silver, but is an alloy of nickel, copper and zinc. The usual alloy mix for fretwire is 18% nickel, 65% copper and 17% zinc.

This close-up of a section of un-dyed ebony fretboard shows several empty slots and one fitted fret.

In the next article in this series …..

In my next article we’ll take a look at fret spacing, tuning, temperament and scale-length.
-----> Link to Scale length, fret positioning, tuning and intonation

By Terry Relph-Knight

My thanks for their help in writing this article to – Taylor Guitars, to C.F. Martin & Co., to Stewart-MacDonald and once again to Frank Ford of Gryphon String Instruments.