|Nut slots (with
principles that apply to bridge slots as well)
Here's a gnat's-eye view at the face of a nut as seen from the leeward side of the second fret. The slots for these two strings are cut so that they completely support the string.
The sketch above
relates to fretted instruments, but the principle is
no different for violin family and other unfretted
instruments. I'll try to explain the clearance in a
Here's an idea of
how it works on a bass:
shimmed-up mess of a nut that has all the problems:
These slots are all too deep, but the B is still so high it doesn't play in tune, so someone shoved a piece of ebony under it to try and correct the intonation. Big "Ugh" for this one.
comment on certain strings (mandolin A strings,
guitar G strings) being more troublesome, always
seeming to go out of tune during play. Mandolin A's
are always the most troublesome because they have to
make compound bends from the nut: back and to one
side, and the length from the nut to the post being
the other important factor. And being plain strings,
they tend to bind if the slots aren't cut right.
(The D's, being wound, tend to refine their own
When you tune,
you always tune up to a note, never down,
And with a badly
cut nut, when you tune up, the tension on the length
of string between the nut and the string post is
greater (per unit of length) than the part you
actually play, that's between the nut and the
bridge. It's about friction in that slot. After
getting the pitch just right, a bit of actual
playing works the string, making the tension on both
sides of the nut equalize, and voilà: you're out of
tune in mid-phrase. It has nothing to do with the
tuning machines, which people just love to
blame, but everything to do with setup, particularly
how precisely the string slots at the nut are cut.
A quick word
about creaking guitar G strings: this issue is
fading as elephant ivory nuts are fading. Bone is
superior to ivory for a nut material because it's
harder and burnishes better. Ivory is soft and
actually registers the imprint of string windings.
That irritating creak is the sound of the windings
skidding over grooves impressed inside the nut slot.
Once again: setup is everything. (You can resurface
string slots in
an ivory nut with pearl or bone, if you like.)
How do you easily determine the ideal height of the string slot in the nut? (Hint: start with ⓵)
is a superb straightedge when it's under tension.
This assumed the frets are really true and level.
The sketch below
illustrates how - and how not - to shape a slot for
Left: the nut is
too high. When the string is way below the top of
the nut, you have great difficulty telling whether
it's seating properly.
Next: a slot that's cut with a saw has a roughly flat bottom and also affords poor acoustic coupling. Saws seldom match the precise width of the string, which will roll side to side in the slot.
will work their way down a v-cut, often bottoming
out on frets (or the board, as the case may be with
fretless instruments). The signal transfer is
compromised because of the limited contact, and the
string sizzles on the fret or the board. They also
tend to bind and squeak. They can ruin your day.
Right: the slot
really fits the diameter of the string, does not go
above the halfway point of that diameter, and leaves
the string a trace of clearance above the fret or
the unfretted board surface.
If you hold any string down on any fret of a well-setup instrument, you'll see that same preferred clearance at the next fret up. I'm reluctant to assign a measurement—it's very little.
Before going further, here's how to
correct a string slot that's too low. Often it's
wiser to repair a blown slot than it is to replace
the whole nut.
Quick fixes like some kind of dust
(bone, acrylic, baking soda) with superglue are
really temporary. It takes little more effort to
implant a little patch of bone (or even pearl)
into the nut and recut the slot. It's as good as
the original, and if done well, is quite
I have a couple
of saws I use for widening and deepening in
preparation for an implant. One is a fine hacksaw
blade in a short handle, which is for wider strings.
It leaves a nice flat-bottomed slot. The other is a
backsaw such as one would use to cut fret slots,
which does the same and is good for finer strings.
Avoiding hitting the first fret, assuming there is
one, I cut down below the blown slot, sometimes
almost to the board itself, angling the saw back a
bit. Then I prepare the piece of bone (or whatever:
ebony for a violin or cello) by carefully filing a
piece of the material with a fine flat file until it
slips snugly into the slot. I usually use old saddle
scraps for this. A drop of CA and a tap and it's in
Trim and dress
the nut as if it was new and uncut, then cut the new
I prefer to shape my slots in the shape of a horn's bell:
The point of this
is to offer a smooth surface for the string to
travel from the tuning machine to the critical point
of final contact at the front of the slot, where it
is held firmly to define the end of the vibrating
Strings have to make a compound bend at the nut, and to make tuning easiest while ensuring complete firm contact at the front of the slot, this horn bell shape makes certain the string glides smoothly, no matter the angle of approach. Here's a treble side view:
The bell here is imaginary. The nut is in yellow, the fingerboard is dark brown. The string is the green line, and the tuning machines are off to the right somewhere. Notice that the string connects with a smooth curved surface, no corner or edge. Whether the string is coming from the top or the bottom of the string post, it will slide smoothly into the nut slot. The string is in complete contact with the front 30% of the nut. There's plenty of substance there to keep the string from sawing its way deeper into the bone.
Here's the same slot seen looking straight down from above:
The string's other curve, from, say, the farthest peg on the bass side of the headstock, also elides with the inside of the bell-shaped slot, guided gently and firmly to the front where it's held firmly by its own tension inside the confines of a well cut slot.
If the slot isn't
properly angled back, several problems can arise. If
it's too flat (some repair books actually advocate
this!) the string soon wears away the front of the
slot and the functional point of contact is as much
a 40% of the width of the nut back from the front
edge, which can cause the note to ring poorly
(because it's vibrating along a surface, not held to
a point) and perhaps cause intonation problems. This
If the slot is
angled back, but left a straight line, it will bind
on the back edge, and the front edge will wear down
from playing and the string is at risk for sizzling
on the first fret or on the surface of the board.
This is also bad:
The precise shape
of the slot at the front edge is extremely important
for sound quality, stability of the setup, and
More on bridges
in due time, but the principles here apply to bridge
slots on the viol and violin families, guitars,
mandolins, and so on.