If you can do a good job identifying your basic diatonic intervals (unison, major 2nd, major 3rd, perfect 4th, perfect 5th, major 6th, major 7th, perfect octave), the next logical step is to move on to hearing the distinctions between the different interval qualities:
- major/minor 2nd
perfect/augmented 4th and perfect/diminished 5th
In the beginning of your relative pitch ear training, it helps to relate intervals to the sound of a major triad (major 3rd and perfect 5th). For example, a perfect 4th would be a half step up from the triad’s third and a minor 6th would be a half step up from the fifth.
Another trick is to think of familiar tunes. The first two notes of ‘The Wedding March’ are a perfect fourth and the first two notes of ‘The Godfather Theme’ are a minor 6th.
Remember to test yourself harmonically (both notes played at the same time) and melodically (one note played after the other). Also, when practicing your intervals melodically, remember to play them going up and going down.