Introduction
The geometry of the classical guitar is fundamental to its sound, its playability, and its durability over time. Every parameter interacts: the height of the strings above the fingerboard, the neck angle, the thickness of the fingerboard itself, the precise depth and level of each fret. Getting these relationships right is what separates an instrument that plays well from one that merely sounds good in the open position.
Reference
The key
parameters
| String height at 12th fret (treble) | 3 mm |
| String height at 12th fret (bass) | 4 mm |
| Saddle height (treble) | 10 mm |
| Saddle height (bass) | 11 mm |
| Fingerboard thickness | 5 – 7 mm |
On saddle height
Saddle height depends on the construction of the instrument, but as a general rule it should not exceed 11 or at most 12 mm. A nut that is too high generates excessive pressure on the soundboard. I aim for 10 mm on the treble side and 11 mm on the bass.
Phase I
Dressing
the fingerboard
Two approaches to neck geometry
The luthier has two methods for achieving correct geometry. The first — more historical, common in 19th and early 20th century guitars — places the neck in the same plane as the soundboard, then dresses the fingerboard to obtain the desired neck angle. The second, more modern approach, advances the neck one or two degrees above the plane of the top during assembly, allowing a fingerboard of uniform thickness throughout. This method requires an adjustment after the 12th fret to accommodate the angle between neck and body.
Flattening with a jointer plane
It is essential to ensure the plane irons are well sharpened. The fingerboard is flattened with a jointer — a plane with an extended sole — which bridges local irregularities and ensures a truly flat reference surface over the full length.
Checking the neck angle
To adjust the neck angle without stringing the instrument, a simple method: lay a straightedge along the fingerboard and check the gap between it and the top at the position of the future saddle. This gives the neck angle precisely.
The bass-side slope
A hand plane creates a slope on the bass side, lowering the future saddle height. This gives 1 mm of difference between treble and bass, rather than 2 mm for a flat fingerboard — working toward the target measurements above.
Relief after the 12th fret
Beyond the 12th fret, a slight slope is created across the full width of the fingerboard to prevent the strings from buzzing against the upper frets. Under string tension, the neck is pulled forward — so the slope depends also on the flexibility of the wood. For ebony fingerboards, a slight slope is sufficient. For rosewood or a less hard wood, it should be more pronounced.
Phase II
Slotting &
fretting
Fret spacing templates
Calculating fret divisions by hand introduces too many opportunities for error. Most luthiers use templates available from specialist suppliers, which give precise slot positions for a given scale length. A triangular file then creates a small chamfer on each side of the slot — this prevents chip-out when frets are replaced in future.
Deepening the slots
The slots are deepened with a specialised saw whose cutting depth is adjustable. The last fret is inserted from the side using a hammer. Frets are seated with a plastic-faced hammer, with animal hide glue applied to each tang.
Checking level with a fret rocker
Once three frets are in place, a small tool called a fret rocker is used to check their level: it sits across three adjacent frets and is rocked side to side. If it moves, the central fret is higher than its neighbours and needs another blow from the hammer. This operation is repeated for every fret.
Phase III
Levelling &
polishing
Levelling with a diamond stone
One of the final steps in building a guitar. The fingerboard and soundboard are well protected, then a diamond stone is used to level all frets uniformly. The excess material is removed by passing repeatedly along the full length of the fingerboard until every fret is at the same height.
Recrowning
After levelling, the crown must be restored — each fret must be rounded again. This is critical: a flat fret shifts the contact point toward the saddle, sharpening the note. A specialist fret file is used to restore the radius. A second small file rounds the ends of each fret. Sharp edges are then removed with fine sandpaper.
Polishing
Each fret is polished with very fine steel wool until it shines. A small amount of drying oil is then applied to nourish and protect the fingerboard wood — and the work is done.
The geometry of the classical guitar is a fundamental parameter — for its sound, its playability, and its longevity over time.