An interval is simply a relationship in pitch between two given notes. The two most basic forms of intervals are Major (M) and minor (m) intervals. below is a chart of all the basic Major and minor intervals. Underneath the text is a short MIDI file of what the interval sounds like.

Perfect Unison (aka Prime)
minor 2nd (m2)
Major 2nd (M2)
minor 3rd (m3)
Major 3rd (M3)
Perfect 4th (P4)
Perfect 5th (P5)
minor 6th (m6)
Major 6th (M6)
minor 7th (m7)
Major 7th (M7)
Perfect Octave (P8)

Alterations upon these intervals are also used. All intervals can be augmented or diminished. An augmented interval raises either a Major or a Perfect interval by one half step, but maintains the same letter name. A diminished interval lowers a minor or Perfect interval by one half step, and maintains the same letter name.

1. Major--interval as is
2. minor--a half step lower than a Major interval
3. diminished--a half step lower than the minor interval
4. augmented--a half step larger than the Major interval

It is very important that emphasis is placed on the fact that the letter names do not change. When dealing with diminished or augmented intervals, double sharps and double flats are very common. It is important to realize that these intervals may contain the same pitches as other intervals. The difference lies in the letter names. For example, an augmented 5th (C-G#), when played on an instrument, will sound the exact same notes as a minor 6th (C-Ab). However, these intervals are not the same. They are called "enharmonic" intervals, meaning that the pitches are the same while the interval is different. It is important that you maintain the letter names when augmenting and diminuting intervals. A comparison could be drawn between enharmonic intervals in the "musical language" and the homophones of the English language.

The inversion of an interval means that the lower note becomes the higher note, and the higher note becomes the lower note. It is kind of like flipping the interval over. You will find that when you invert intervals, the interval between the notes will change. The following table will illustrate my point.