Intervals, explained

Finding each interval on the neck

On one string the table above is literal: the semitone count is the fret distance. A major 3rd is four frets up, a perfect 5th seven, an octave twelve. That is accurate but slow, because nobody solos by sliding a hand seven frets to find a fifth. The shapes that matter cross strings.

Standard tuning puts a perfect 4th (5 semitones) between neighboring strings, so the same fret on the next higher string is a 4th. From there, count frets: one fret back is a major 3rd, two back a minor 3rd, one forward a tritone, two forward a perfect 5th. Skip a string and the same fret gives a minor 7th, two frets forward gives the octave. That octave shape is the quickest way to find the same note in another position.

The exception is the G and B pair, tuned a major 3rd (4 semitones) apart. Any shape that crosses from the G string to the B string moves one fret higher on the B string: a 4th becomes one fret forward, a major 3rd sits at the same fret, and an octave from the D string lands three frets up instead of two. That offset is why a shape that works on the lower strings seems to break on the top two.

Practice it by naming, not just playing: pick a note on the low E string, find its 3rd, 5th and octave on higher strings, and say each name out loud. Then check whether you can hear the same distances in the ear trainer.