TOOL
Notched straightedge
Reads the fretboard itself, not the tops of the frets.
| Scale lengths | Separate rules for 24.75 in, 25.5 in and others |
|---|---|
| Notch spacing | Matched to the fret positions of the scale |
| Material | Aluminium or steel, machined flat |
| Length | Long enough to span the full board |
| Marking | Scale length printed on the rule |
| Use | Checking board straightness under string tension or slack |
| Flatness | The rule itself must be verified flat |
|---|---|
| Reading | Look for light under the rule at the middle and ends |
| Relief | Board should show a slight forward bow under tension |
| Diagnosis | Board curved but frets level: adjust the rod |
| Pairing | Use with a fret rocker for individual frets |
| Price band | Low to moderate |
A notched straightedge has slots cut at the fret spacing of a particular scale length, so it sits on the wood of the fretboard between the frets. That lets you see the actual curve of the board, independent of how level the frets are.
It is the tool that tells you whether a neck needs a truss rod adjustment or a refret. A straight board with unlevel frets needs fretwork; a curved board needs the rod adjusted first.
At this stage
Use it before any fretwork and before any setup. It answers the first diagnostic question: is this a board problem or a fret problem?
Suits these instruments
- All guitars and basses
Works with
- Fret leveling file
- Truss rod
- Feeler gauge
- Fret press
- Do not lever the rule against a fret. It is a measuring tool, not a pry bar.
- Remove it before you touch the truss rod so you do not forget it is on the board.
Working notes
- Own the rule for the scale lengths you actually work on. A mismatched rule is worse than none.
- Check the rule against a machined flat surface before you trust a reading.
- Read with a light behind the rule — the gap is what you are looking at.
- Combine it with a fret rocker to find a single high fret.
Related
Action gauge
Reads string height at the twelfth fret without arguing about it.
Feeler gauge
The only honest way to measure relief and nut height.
String winder
Turns a fifteen-minute string change into five.
Capo
Clamps the strings at a fret — and shows up any setup problem.