Acoustic Guitar Neck – A Brief Guide to Function, Woods, Shape, Relief & Truss Rod

Neck of any stringed instrument projects the main body and extends the strings from the body of the instrument to the headstock. Headstock usually consists of geared pegs to tune the strings. Acoustic guitar neck forms the base for the fingerboard, which is a strip of dark hardwood, on which frets are seated. Inside the neck, we have a steel rod, known as the truss rod.

Major functions - Acoustic Guitar Neck

The major functions served by acoustic guitar necks are:

  1. Withstanding string Tension: Neck supports the strings and withstands considerable bending stress, particularly with heavy gauge steel strings. Hence, a strong and hard wood is required to hold everything together.
  2. Resist warping: Ability of any guitar to resist warping allows it to hold a constant pitch. Strings must be at a certain height above fingerboard to provide good playability. Warped neck is almost impossible to finger. Truss rod provides this ability against warping. 
  3. Playability: Size and shape of the neck plays an important role in influencing the playability of any guitar. People with smaller hands prefer narrow and shallow necks, while players with fingerpicking style, bigger hands and fingers, prefer wider and deeply curved necks. 
  4. Tonality: Guitar necks should absorb minimal energy from vibrating strings, so that more energy reaches the soundboard through the bridge. This will ensure that type of tone produced by the guitar, is defined by the body only. Hard and dense wood ensures this.

Choice of Woods for Neck

Following are the common choices of woods for the acoustic guitar neck.

  1. It is clear from the above section that wood for neck construction must be dense, hard and suitable for carving. Hence, quarter sawn Honduras Mahogany gets used on necks of all good steel string guitars for maximum stability and strength to weight ratio.
  2. Inexpensive guitars, use Nato as an alternative, which is a Mahogany like wood with origins from South Pacific.
  3. Guitars with Maple or other light colored bodies often use Maple necks. As Maple is less stable, it is common to cut them lengthwise in two or three pieces. These pieces are  interleaved with ebony, rosewood or some other hard wood by gluing them. While gluing, the grains are kept in different directions, to impart strength and warp resistance. Many times, dark woods are chosen for interleaving to provide visual contrast.
  4. Laminated necks are quite sturdy but heavy. They can be difficult to balance in your lap unless used on large bodied guitars, where body can balance the neck weight.
  5. Some classical guitars still use Cedar. Cedar is able to withstand nylon string tension but not can’t withstand tension of steel strings.

Neck Contour

Neck can have full round contour or a slender one around their backs. 

  1. Full round necks, also referred to as chubby necks by some, are stronger, due to more hardwood in them. 
  2. In earlier times, back of the neck used to me more triangular than round, referred to as vintage triangular style. These necks allowed you to loop your thumb around the fingerboard, placing your thumb over the top, making it very easy to play the guitars. Some luthiers have launched some models with triangular necks in recent times.
  3. Most of the modern guitars have metal truss rods, reducing the reliance on wood for strength, as in full round necks. So slender or less rounded necks have become a norm. Slender necks allow fast movement along the length up and down. 

Relief in a Guitar Neck.

Neck of a traditional guitar is not straight and is in fact little warped around the sixth fret. This dip makes the upper frets more easily playable. It also helps to avoid buzz at the frets in the area where the neck joins the body of the guitar. This little intentional warp is known as relief.

Requirement of actual relief on any guitar also depends on playing style and scale length. 

  • Players with moderate touch require less relief, while heavier touch requires more relief.
  • As scale length of any guitar decreases, less relief is required.

Guitars are provided with truss rods to adjust relief. But, It is not recommended for beginners to adjust the truss rod by yourself. 

In modern guitars, truss rod is usually fixed at the nut end and is adjustable at the body side. It can be tightened or loosened with nut driver or hex wrench. In earlier guitars, the configuration was the opposite, with fixed end on the body side.

Advent of Truss Rods

In times before the use of truss rods, relief was dealt like warping. Relief adjustment was carried out by heat treatment of neck or by planing the fingerboard.

Around 1920s, as the strings changed from gut to steel and nylon, neck came under tremendous pressure and started warping. Initially, luthiers gave up Cedar for stronger woods to overcome the warping with marginal success. 

Martin introduced long ebony strip in the groove cut in the neck to add strength. In few years, Ebony got replaced by steel reinforcing rod. Reinforcing rods were T-shaped bars, U-Channel bars, Hollow square tube before evolving to Adjustable Truss Rod. These truss rods are placed inside a U channel bar with square dowel outlay. Titanium and graphite is used these days.

Guitars introduced with thinner neck contour particularly required truss rods to compensate for lack of wood to give adequate strength. 

Some Twelve string guitars even have two truss rods. In these guitars, tension is much stronger on the bass string side, justifying the use of two side-by-side truss rods. 

Relief by Truss Rods

Main purpose of truss rods is to prevent warping, as against the common misconception that these rods can make the neck straight. They basically regulate relief.

Truss rods have an arch between fifth to seventh fret, giving neck a slight forward bow. This subtle U shape provides more space around middle frets, allowing the strings to vibrate more freely. 

Most manufacturers will provide little relief in the neck. If you feel, the relief to be excessive, you can tighten the truss rod to reduce it. But if the built-in relief is less for you, you can’t do anything with single adjustable truss rod to add it. 

Also, truss rod cannot provide solutions for 

  • Neck set and nut or saddle height problems
  • Warping in other areas of neck
  • Reverse Warping.

Functioning of Truss Rod

Truss rods exert a compressive force of about 20 pounds in opposition to string tension, which is about 150 pounds in most guitars. For the truss rod to provide the required compression in the desired direction, it is important to ensure that

  • Truss rod is seated deeply in the neck.
  • Its channel is firmly glued over with strongly built square dowel.
  • Fingerboard must be solid with more wood and glued properly..

The compression by truss rod in the wood below is provided by pushing against the dowel and fingerboard above. A truss rod too close to the fingerboard, flimsy neck or less wood above the rod will not yield the desired results. In most cases, tightening the rod will reduce relief and loosening increase it.

Neck Warping

Neck warping can occur in the direction of relief or in opposite direction. This warping can be in the same area, where relief is provided or in other areas up and down the neck. In extreme cases warping can be in more than one direction, though rare.

Warping in direction of the relief, a concave distortion, is called as the forward warp or forward bow or simply bow. Necks with metal truss rods are less likely to warp, but in extreme circumstances can warp.

Warping in direction, opposite to the relief, or convex distortion is called as back bow or reverse warp. 

In case of forward bow, you may have buzzing problem in 1st to 5th and 12th to last fret, as string height will be less in these areas. In reverse warp, you may face buzz in the middle frets from 6th to 12th.

How to check or measure the relief

Looking for string height in the middle frets (5th to 7th) will give you an idea about the subtle bow. To diagnose or measure the relief, do the following

  1. Place a Capo on the first fret. This will eliminate nut from the consideration.
  2. Hold down the the low E string on the fret where neck joins the body. 
  3. To check relief, you can insert standard business card in the middle frets. If business card is unable to pass easily, you have a reverse bow. If the card is able to pass with big gap visible, you have forward bow. 
  4. To measure the relief, you can use feeler gauges.

Relief Adjustment

It is recommended to get the relief adjustment done through the repair shops. But if you still want to try it yourself, give only one-eighth of the revolution of the nut at a time. Wait for few hours before giving another one-eighth turn, if required. Tune the strings to normal pitch while doing these adjustments.

Warp Treatment.

Use of truss rod for warp treatment is ineffective, unless it is in the relief area and warping is very limited.

For moderate warping, neck is clamped in a straightening jig and heated by infrared lamps for two or three days.

In extreme warping cases, frets are removed and fingerboard is planed to compensate for warping. Cases of twisting and skew around the length are also treated by fingerboard planing.

Neck joints

The three most common ways to to join guitar neck and body is with a Dovetail joint, Mortise & Tenon joint and Bolted Necks. Dovetail and Mortise joints are also sometimes called as Set joints. 

  1. Traditional method in Acoustic guitars is to use Dovetail joint. Dovetail tenon in the heel fits into a mortise cut into the heel block. Heel block is also called as the head block. It si solid piece of wood mounted directly under the fingerboard. Heel block is visible through the sound hole.

  2. Straight cut Mortise and Tenon joints. These have straight cuts at 90° instead of angled joints as in case of Dovetail joints.  

  3. Bolted On type necks are simply bolted through the heel block and attached using butt joint and bolts. Typically four or six bolts are used in this joint. This joint is easiest to assemble and adjust out of all types.

Earlier bolt on type guitar were associated with inexpensive guitars. Now even reputable manufacturers use this joint on their good instruments. Even with expensive guitars using other types of joints, string pressure takes its toll with passage of time. This requires the neck to be dismantled and reset every 20 - 25 years. Bolted on necks are more conducive to these resets.

  1. Bolt-on necks are very common on solid body electric guitars.

  2. Dovetail joints are the standard on archtop guitars.

  3. Classical guitars use either (a) large sized mortise and tenon joint on a large heel block or (b) Combination one piece heel and heel block, In this arrangement a groove is provided to mount guitar’s shoulders.

Conclusion

In this article we have introduced functions of guitar neck, neck material and contours, relief, truss rod and its adjustment, warping and its treatment and neck joints. In our next article we shall focus on fingerboard and frets. We request you to write to us for any clarifications.

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