Watch the Meet the Intervals online guitar lesson by Kai Eckhardt from Atomic Bass: Reactive Intervals
In the music of our western traditions we distinguish between 12 basic intervals. The simple definition of an interval is the space between two pitches. Even though possibilities appear as infinite, there really are only 12 repeating intervals in various positions on any well tempered instrument. (Fretless instruments and instruments tuned to Arabic scales offer another set of possibilities belonging to a different field of study). We are now going to learn the names of the basic intervals, to play them on the bass guitar and to recognize them by ear. Take a look at my drawing called "Meet the Intervals" and see how all of them are present within the space of one octave from C to the next C higher up. If you start on any other note and span across an octave (C# to C#, D to D) their names remain the same as they are always relative to the tonal center you happen to choose.
We shall begin by placing our index finger onto the E string between the 7th and the 8th fret to play the root C. Then we place the ring finger on the D string between the 9th and 10th fret to play the octave C. Now we strike both notes at the same time by using the thumb of the right hand on the E string and the index finger on the D string. This is the sound of the interval called the octave. The word refers to the number 8, which is the 8th note of the major scale counting up from the root. Now drop the higher note down by successive half steps, one fret at a time while you say the name of the occurring interval. This is how you sweep through all of the intervals systematically and get to know them for the first time.