- Datum / witness point
- The physical reference actually touched by the string, fret, straightedge, or measuring tool.
- Chord and sagitta
- Chord is the straight endpoint-to-endpoint width; sagitta is the perpendicular center depth to the arc.
- Spread
- An overall center-to-center distance unless a tool explicitly says edge-to-edge.
- Projection distance
- The horizontal run used with a measured rise to calculate an angle.
- Break angle
- The side-view angle between the string segment and the reference plane at a witness point.
- Calibration
- A comparison between a commanded printed length and its physical measured length.
Never mix inches and millimetres inside one entry list. Repeat measurements and preserve the raw values before rounding.
Scope of this reference
Define chord, sagitta, spread, datum, projection, break angle, and calibration measurements. Use Luthier Geometry Measurement Definitions to make a repeatable decision for this named task, not to impose one specification on every instrument.
Practical example
A radius calculation using chord and sagitta fails if the straight chord and perpendicular center depth are confused. Likewise, a neck-angle projection must use rise and run from one side-view datum, and a print calibration must be confirmed on paper rather than on screen.
Record the conditions
For Luthier Geometry Measurement Definitions, write down instrument type, tuning, installed strings, relevant hardware, climate when material, physical datum, unit, tool resolution, and the result before and after change. A result from this reference is comparable only when its method travels with the number.
Verification checklist
- For Luthier Geometry Measurement Definitions, confirm the starting state and exact question.
- Repeat its critical measurements from the same reference.
- Compare its result with direct observation and applicable maker documentation.
- Change one reversible variable where the task permits.
- Retest the named symptom and preserve unresolved exceptions.
Related workflow
- fretboard radius matcher — continue when that page answers the next unresolved measurement or decision.
- neck angle planner — continue when that page answers the next unresolved measurement or decision.
- print actual size — continue when that page answers the next unresolved measurement or decision.
Limits
Luthier Geometry Measurement Definitions cannot inspect hidden structure, material condition, service history, or player technique. Stop this reference workflow for electrical hazards, abnormal resistance, structural movement, conflicting readings, or unplanned material removal.