fretlord is a guitar and bass practice utility that runs in the browser: a tuner with partial-rich reference tones and a microphone-driven needle gauge, and a fretboard that marks any scale in any key. Open the tool. It needs a desktop-sized window (the layout assumes at least 1024x700) and sound.
Four to eight strings, each a pitch class, octave and integer cents. Every string has its own reference tone made of four harmonic partials at 1:2:3:4, so the upper partials beat faster and magnify a tuning error into an audible beat rate. Low strings can be played an octave or two up for speakers that cannot reproduce a 31 Hz low B. Cents are dragged or scrolled one cent at a time, and every change is click-free.
Turn on listening and play a string: a YIN pitch detector finds the fundamental, the nearest open string (or any board position) becomes the target, and a needle shows the error in cents. It holds the last reading after the note decays, turns mint when in tune, names a pitch that is not on the instrument instead of guessing, and warns when a reference tone is sounding into the microphone. The browser asks for microphone permission only when you choose Listen > Microphone; audio never leaves your machine.
Frets 0 to 24 at real tempered spacing, low string at the bottom. Pick a scale and a root and every matching position is marked with its degree, across all octaves. Click a position to hear it through the string's own voice; release to stop.
Tuning presets and scales are plain text files. The bundled scale library has the diatonic modes, pentatonics, blues, harmonic and melodic minor, whole tone, chromatic and a 5-limit just minor. Presets you save in the browser are kept in that browser's local storage (IndexedDB) for this site, so they survive reloads but stay on your machine.
Written in Rust with SDL3 and a small hand-rolled immediate-mode GUI, no audio framework. The same code runs on the desktop and, compiled to WebAssembly with emscripten, in the browser. Bundled fonts are Liberation Sans, Liberation Sans Narrow and Liberation Mono (SIL Open Font License 1.1).