Knowing the key tells you which notes and chords will fit. Here are the by-ear tricks guitarists use, and how Fretnook can listen for you, either live through your microphone or by analyzing a song file you load.
Play or hum along with the song and find the note that feels like it is finished, the one the music wants to land on. That note is usually the root of the key.
Songs often end on the chord of the home key. Find that chord on your guitar and check whether it sounds major or minor.
In a major key the chords built on the 1st, 4th and 5th notes are major, and the ones on the 2nd, 3rd and 6th are minor. If the chords you hear fit that pattern around your home note, you have the key.
If the song feels sad but the chords fit a major key, it is probably in the relative minor. A minor key sits three semitones below its relative major, so A minor shares its notes with C major.
The Ear Trainer listens through your microphone and shows the key and scale you are playing in, with an in or out of scale readout as you drift.
Load a song and Fretnook shows its key, tuning and chords as it plays, so you can see what is happening under the melody.
See the scale on the fretboard for standard tuning, Drop D, Whole Step Down and Open G, D and E.
Loop the hard part and slow a song down without changing its pitch while you work out the notes.
Once you know the key, you know which scale to solo in and which chords will fit a progression you are writing. A pentatonic or major scale built on the key note gives you safe notes to play, and the chords from the same key give you a starting point for a song.
Detection is a guide, not a verdict. Songs that change key, borrow chords from another key, or have very little harmony can confuse any key finder, so use your ear to confirm the result.
After you find the key, set it in the Band tab of Fretnook and pick a chord preset such as a 12-bar blues or a pop progression in that key. The backing band plays the chords while you practise your solos over them.
Fretnook is free and runs in your browser. Your microphone audio and recordings stay on your device.