The bridge supports one end of the strings and the other end is supported by the nut. Many guitars have a bridge that is easy to adjust; most nuts have to be adjusted carefully with properly sized files, a task best left to one that is comfortable working in fine increments. Perhaps the most common cause of tuning instability for most guitars is friction at the nut.
The nut is critical to the playability and intonation of the guitar; if it is too high, the action will feel stiff to play and the lower frets will play sharp. If the nut is located too far away from the first fret, the lower positions will play sharp as well. If the slots are too tight and/or not relieved at the back edge, friction will cause tuning issues. If the nut material is too soft, friction will cause tuning issues and the slot height will eventually wear too low. If the slots are too wide, the string will move laterally and also have a weak, buzzy sound. If the spacing is too close to the fret end bevel or fingerboard edge, fretting notes on the outer strings can be challenging and uncomfortable. Spacing from string to string should be neat and consistent without any over or undersized gaps; I prefer a compensated string spacing at the nut, not equal center to center or edge to edge, but a balance between the two to accommodate the increasing gauge of the strings.
Cutting a nut well requires patience and practice. There is no easy way to jump into it. When I make a nut that is not included with fretwork, I bill based on the hourly shop rate; a nut included with fretwork is priced lower as I am already dealing with nut clearance as part of that work. Typically a Fender style nut is faster to make and complete than a Gibson/Martin etc style as it is faster to fit the blank and complete cosmetically. Preslotted nuts can save some time in the completion process, but still take time to fit properly; I rarely use preslotted nuts, only by request as I generally do not like many of the materials that preslotted nuts are made. The nut is not a good place to cut corners due to its critical function and impact on the instrument’s playability and intonation in my opinion.
If replacing an existing nut, the old one must be removed and the following describes relevant concerns: Nuts that are held captive between a headcap and fingerboard (I.E. a Gibson Les Paul Custom etc) can be involved to remove if they are overly glued by the installer. Many nuts are installed as blanks prior to the finishing of the instrument and care must be taken to remove them in order to minimize finish disturbance. Some finishes are thick and/or brittle and thus difficult to work with. The ends of a replacement nut can be leveled flush with the surrounding area and either highly polished or finished back in to match. I don’t like ugly, uncompleted looking nuts (a pet peeve like poorly done fret ends for me, ugh!), so I shape the tops and ends so as to minimize the feel when playing in first position (F barre chord or F13, moving for behind the nut string bends ala those wild & wacky Tele players etc) and polish up everything to a nice high shine.
I prefer that a nut fit its slot with a good friction fit and be well mated to the bottom and fingerboard end. I glue them as well to insure they stay in place and to maximize sonic transfer. Some folks advocate using white or wood glues; I find these to be soft and rubbery and so I use cyanoacrylate glue (super glue) in the appropriate amount to bond. A neck’s nut slot/shelf/location may not be uniform and can require preparation to fit the blank; again Fender style nuts generally are faster to fit than others. Some nuts do not have any type of captive slot (I.E. PRS guitars) and depend on liberal use of adhesive to hold and transfer vibration.
When a guitar has good fretwork and is properly set up, the clearance over the 1st fret should be slightly more than the clearance of the string fretted at the 1st fret and clearing over the 2nd. A general rule applying to set up and nut height: The harder the attack, the higher the action, the higher the nut. For example: if the 2nd fret clearance is ~.010" for a 2nd/B string, then I would allot ~.015" clearance over the 1st fret. Bigger strings tuned lower require more clearance, so if the 6th is ~.020" over the 2nd fret, I would allot ~.030" over the 1st.
If a string is not clearing the 1st fret when the relief and bridge height are correct (fretwork reasonably good), then the slot is too low.
When one or some nut slots are too low, some people fill and recut the slot - I don't like this method, it will be weaker than the nut material and can grab the string, increasing friction, and wear out quickly – I consider this an amateurish, poor quality fix. If the nut can be removed cleanly, it can be shimmed and slots recut as necessary. There are some issues which discourage shimming a nut and I don’t like this method for nicer condition or high quality instuments.
Many materials have been used for nuts thru the years: bone of many animals, hardwoods such as ebony, ivory, nylon 6/4, phenolics, corian, PVC, various plastics (I.E. Fender’s Cyclovac, describe as “bone” in color), various formulations of graphite, “Soapstone” (DuPont Delrin) brass and aluminum. Plastics are cheap, can be injection molded preslotted as a labor saving device, but can be soft and outer strings can break off corners. PVC is particularly soft and often seen on the cheapest guitars. While very strong along intended grain alignment, graphite composites can be soft as a string bisects it, placing force in a narrow area, and can have a dull, dampened tone – Delrin exhibits similar qualities. Corian, a mainstay of Gibson since at least the 1990s, varies in hardness. Brass was popular in the 1970s; it can bind steel strings sometimes. Danelectro used aluminum, an interesting material, but can be soft in the string slots as well. Ivory, unless fossilized, is not only generally illegal, but also on the soft side.
My preferred material for nuts is unbleached bone. I find unbleached bone to be hard. Compared to bleached bone, the unbleached seems to retain some natural suppleness (bleached bone can be chalky, dry, brittle and have significant open pores). Bone is a byproduct of the meat packing industry; I eat meat, wear leather, use hide glue and am happy to use bone as well and not waste a resource. An argument against bone is that as a natural material, it is inconsistent. The solution to that is, just like in choosing wood to build an instrument, choose good pieces of bone! I discard blanks that do not meet my standard. Some folks advocate materials with internal lubricating ingredients; I find these to be soft and generally sound duller and wear quicker than a harder material. I find that a hard piece of unbleached bone with properly cut and polished slots, lightly lubricated, supports the strings, sounds good, wears well, and allows the strings to travel thru the slot in the course of playing. A string is a hard, polished and precisely dimensioned materials and it needs to reside in a mated, hard, polished slot not unlike moving pieces in a machine or motor working with and against each other.
Click for enlargement/captions:
Bone is fine to use for players that bend frequently or use a non double locking (Floyd Rose style etc) tremolo. While nothing will take heavy whammy bar use & abuse/divebombs like a Floyd, the properly fitted, slotted, polished, lightly lubricated unbleached bone nut will behave well with moderate non locking tremolo use. Remember that a tremolo is really a system and that best results are obtained with not only a friction minimizing nut, but also stable, returnable pivot points and proper stringing technique, if not locking tuners ala Sperzel Trimloks etc.
The only exception I have found to unbleached bone for nuts (and also acoustic guitar saddles) is Graphtech’s TUSQ material. TUSQ is not as hard as bone, but has a really nice bright, open sound. Some acoustic guitars actually sound better on an individual basis with TUSQ instead of bone saddles.
I only use exotic materials like fossilized walrus tusk etc upon request. While the materials are pretty to look at, I honestly do not hear a great positive difference in tone to justify the cost over bone.
Nut slots should be lightly lubricated occasionally. Over the years I have tried pretty much everything – Teflon oil gun lube (messy), graphite powder and Teflon powder (can coagulate over time, begin to increase friction as it is something else in the slots), Chapstick (basically wax, can attract and retain particulates like oils), Big Bends Nut Sauce (expensive). I prefer to use and have had a lot of success with regular old drugstore petroleum jelly, Vaseline. I find a gel lubricant works best, even though it too can attract and retain dirt over time. Vaseline is clear so it is easy to see when it is dirty; the solution is to use something small and soft, like the edge of a business card, to push the old gel out of the slot an apply new, usually during a string change. I have a 7.5 oz jar of $.99 petroleum jelly at the bench with a toothpick as applicator and have only used half of it in fulltime repair in 4 years, not bad huh?
Have you ever tuned up a guitar to a tuner and it seems generally in tune, but open chords and some low notes sound sour? Then you retune to favor a chord or chords? Open D on a guitar is especially rank as it has a major third as its top note, an interval that can be sharp in modern Equal Tempered Tuning anyway.
There are examples where an instrument will have good fretwork, proper relief, and a nut that is cut properly, not too high, and using good strings, yet the notes in the lower frets will be sharp, even when intonation from roughly frets 5 thru 12 will be decent. This cause of out of tune lower fret positions is the nut location relative to the scale length.
Some manufacturers do not locate the nut in the right place according to the scale length; some, like Fender, place the nut at the Rule of 18 or Twelfth Root of 2 scale length spacing and do not allow for increased tension from string deflection at the playable end of the string, where the increase in tension is displaced less equally across the length than notes played in middle positions (Rule of 18 and Twelfth Root of 2 describe two different methods for calculating western 12 note scale length divisions of a string). Some manufacturers like Paul Reed Smith and Taylor, have moved the nut forward towards the 1st fret to alleviate lower position sharpness with good results.
Once upon a time, nut location was modified by cutting a specified amount off the end of the fingerboard and moving the whole nut forward. That is a permanent mod and probably not the best idea for some instruments or resale. My preferred method for correcting nut location is to make a one piece shelf nut. A shelf nut is more involved to make as it must not only fit the neck nut slot/location, but also the overhanging shelf must match the radius of the fingerboard and fit tightly. Shelf nuts are part of the Buzz Feiten Tuning System as Buzzy sough tto correct the nut location’s impact on intonation and temperament. A shelf nut can be used with any desired temperament if fitted for it.
There are other nut compensation methods, notably the Earvana nuts. I have had great success with straight leading edge compensation, but occasionally have altered specific string locations to taste (depends on instrument and application). Earvana comes in two versions, the OEM as used by Ernie Ball Music Man, and the retrofit version. The OEM version is OK, I prefer bone so I am not wild about the material Earvana uses; the retrofit nut is two pieces and the material is not up to the design in my opinion.
Below are some pictures of shelf nuts I’ve made (click for enlargement/captions):