The diagram below shows the C note on the bottom. Every other note has a certain distance from the C note. That distance is an interval. The exact distance has different names. Look at the F note; it is the major fourth because it is the fourth note apart of the C major scale. You would count C as 1, D2, E3, and F is the fourth (4). If you heard the notes C and F played back to back, the interval difference between the two notes is a major fourth. When you hear two notes played, it is almost impossible to know the exact note that it is. For example, most people cannot hear a tone and immediately know that is a Bb (flat). However, when you hear notes played, it is possible to figure out the distance between those notes. You could hear single notes played back to back or two notes played together.  

The example diagram is hearing the C note first and then the distance away from that note, but normally you will not know the first note is a C; you will just hear a first note and then another note. The intervals between the two notes will sound the same to us regardless of what the exact notes are that are being played. In this example if you heard the first note (assuming C) and then an Ab (flat), you would count C as 1, D2, E3, F4, G5, and Ab (flat) as the 6th. You can tell that the second note did not sound as high as an exact 6th major key away from the first, but it was higher than a 5th, so it must be in the middle…a minor 6th. Don’t worry, this takes practice to figure out distances. Remember you won’t know the keys that are being played, but you want to hear the distance in the notes. Imagine hearing a D# (also known as Eb) and the next note you hear is a G. The distance is a major third. Eb is the first note, F is the second, and G is the third note apart of the Eb major scale. To understand intervals, you must understand the 12 major scales. Now imagine the first note you hear is D. The next note you hear is G# (or Ab). The distance is a tritone. A tritone is between a major fourth & fifth. D would be the first note, E second, F third, G fourth, G# tritone. The A note would a distance of 5 away from D, which is called a perfect 5th. Make sure that you understand the 12 major keys or scales. You will need to know that information to help you in this.