The Remaining Piece in the Playability Puzzle
Fingerboard relief is one of the least understood and the most obscure variables in a guitar, yet it has a profound influence on playability.
In previous blog posts, we discussed how action and neck shape and size contribute to left hand playability. Now let’s examine fingerboard relief — the last piece of the puzzle that can make a guitar play like a dream or a nightmare. Let’s begin with the take home message and end with the details.
The Take Home Message
What is fingerboard relief and why is a tiny amount a good thing?
Relief refers to the upward curvature of the fingerboard (and therefore plane of the frets). In the exaggerated diagram below, consider the vibration of a string which sweeps across the guitar in an area shaped like a square parabola:
Luthiers introduce small amounts of upward curvature into the fingerboard to resemble the shape of the strings’ paths. In doing so, string action heights can be lowered, which will make your left hand very happy.
Relief is usually most pronounced beneath the 6th string, which oscillates considerably more than all the others, and least (or non-existent) beneath the 1st.
The correct action height of the strings (as adjusted by the height of the saddle) and an appropriate amount of upward fingerboard curvature work together like complementary puzzle pieces, providing buzz-free notes with minimal left hand effort. The optimal amount of curvature is surprisingly small — between 0.2–0.5 mm deviation from a dead-straight fingerboard. To give you an idea of scale, a credit card is about 0.7 mm thick.
(Read the details in the latter half of the blog post if you want to better understand how relief is introduced and assessed)
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Why is too much relief a bad thing and what causes it?
Beyond a small amount of upward curvature, excess concavity in the fingerboard means that fretting requires pushing the string an extradistance before it contacts the fret. More than approximately 0.5 mm of relief under the bass strings and anything more than 0.3 mm under the trebles can make your left hand work too hard.
The cause of excessive fingerboard relief is often insufficient neck strength to counteract the tension of the strings.
If you are considering buying a guitar…
…and the string action heights at the 12th fret seem reasonable, but the left hand still feels taxed, the guitar may have excessive relief. If the relief is astronomically high, simply sighting down the edge of the fingerboard along the length of the 6th string will reveal an arc.
To assess the actual amount of relief, use the diagnostic test below. The relief should ideally be close to zero beneath the 1st string and no greater than 0.5 mm beneath the 6th.
It’s generally wise to walk away from a guitar with excessive relief. On the other hand, the absence of relief may not be a deal breaker for some players so long as the left hand is comfortable. It really depends on the nature of their right hand attack and whether it is prone to making the strings buzz. Like many things in guitar, it comes down to player preference.
If your current guitar has excessive relief…
…consult your local luthier. There are remedies that could improve your left hand playability, but they require significant work costing in the neighborhood of $500 or more.
Now for the details...
Check your relief with this diagnostic test
Step 1: Place the guitar on a table with the 6th string facing you. Place a capo on the 1st fret and then fret the 6th string at the 19th fret with your right hand.
Using the fretted string segment as a straight edge, look for the gap between the top of the frets and the bottom of the 6th string as shown in the left panel of the diagram below.
The gap, which is the relief, should be largest near frets 4–6.
As a rule, the largest gap you can spot should be between 0.2–0.5 mm for string 6, and progressively less for the others. You can estimate the size of the gap by sliding the edge of a credit card next to the gap. The card is approximately 0.7 mm thick, so if you can slide the card into the gap without moving the string, you definitely know you have excessive relief.
On the other hand, if the gap is small, it can be difficult to see. Try hitting the string above one of the frets with a left hand finger while still fretting with the right hand finger at the 19th. You will hear a distinct “plink” that signifies the presence of a gap.
Step 2: Turn the guitar around so that you can get a good look at the 1st string while you repeat the procedure for the 1st string. There should be no or extremely little relief under the 1st string, as shown in the right panel of the diagram above. If the relief on the 1st string is more than approximately 0.2 mm, you have excessive relief.
How do luthiers create relief?
Luthiers typically create relief by combining 1) the natural bending of the neck caused by string tension and 2) the planing and sanding of the fingerboard in a strategic location. The result is a distribution of relief depicted in red in the photo below.
This spatial distribution adjusts for the larger movements of the bass strings compared to that of the trebles. It’s unnecessary and uncomfortable for the left hand to have relief under the 1st and 2nd strings.
Beyond approximately the 9th fret, the action heights are higher so the distance of the strings above the frets is already sufficient to prevent buzzing so no relief is required in this region.
Relief is set by the maker during construction and is typically not adjustable unless the neck has a truss rod. Weak necks can continue to bend over time and explains why vintage guitars often develop excessive relief.
What does a truss rod do?
The reason why steel string guitars have truss rods is to counter the string tension that is roughly twice that of a classical. The truss rod is tightened to straighten the inevitable curvature of a steel string neck.
The reason why steel string guitars have truss rods is to counter the string tension that is roughly twice that of a classical. The truss rod is tightened to straighten the inevitable curvature of a steel string neck.
Conclusion
I hope this article cleared up any confusing surrounding fingerboard relief!
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In the meantime, if you're interested in fingerboard relief on the classical guitar, I can only assume you have some experience under your belt as a classical guitarist.
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