This post is #2 in my series of music theory lessons. My feeling is that music theory resources on the internet are fairly scattered and typically not for beginners. Theory lessons shouldn’t make anyone run away screaming or scratching their heads, but good luck prying through Wikipedia’s music theory knowledge base, which explains things in thorough, but often highly confusing ways. Much of what I talk about in my Compositional Analysis series requires a basic understanding of theory, but anyone who is interested should be able to read along. I will be using these posts as references for that series. If anything is not clear, feel free to leave a comment.

The distance between two notes is known as an “interval.” Each interval has it’s own name, and these names are useful for analyzing, understanding, and talking about music. I’ll explain why in a moment. But first, here are the 13* basic note intervals in order, with examples starting from C:

• Perfect Unison (C to C – must be identical pitches, no octave displacement)
  
• Minor 2nd (C to Db)
  
• Major 2nd (C to D)
  
• Minor 3rd (C to Eb)
  
• Major 3rd (C to E)
  
• Perfect 4th (C to F)
  
• Tritone (C to F#)
  
• Perfect 5th (C to G)
  
• Minor 6th (C to Ab)
  
• Major 6th (C to A)
  
• Minor 7th (C to Bb)
  
• Major 7th (C to B, a.k.a. the “Leading Tone”**)
• Perfect Octave (C to C – displaced by an octave up or down)
  

Listen to these intervals.

As you can see, each of these intervals are classified as major, minor, or perfect except for the tritone. In a simple world, it would follow that major intervals come from the major scale, minor intervals come from the minor scale, and perfect intervals are present in both. But it’s slightly more complicated than that. You may notice, for example, that the major 2nd is present in both the major and minor scales and the minor 2nd isn’t in either scale (I’m referring to intervals from the root, not intervals between the other notes in the scale).

In order for an interval to be considered “perfect” it must meet two requirements:

1. It must be present in both the Ionian (major) and Aeolian (minor) scales.
2. When inverted***, that interval must be present in both the Ionian and Aeolian scales. Though the major 2nd interval is present in both Ionian and Aeolian scales, it is not considered perfect because it inverts to a minor 7th–an interval which is only in Aeolian. Calling it a major 2nd works out nicely because it means that all major intervals invert to become minor intervals and vice versa.

So who cares whether an interval is major or minor? Why not just have a unique name for everything? Why not just call them 1, 2, 3, 4, 5 […] 11, 12, 13? The reason is because chords are built in stacks of thirds and the types of thirds which make up a chord determine its very essence.

There are two main types of chords: triads and seventh chords.

Triads

Triads consist of three notes stacked in intervals of thirds moving up from the root.**** Check out a G major chord: G, B, D. That’s a triad. From G to B, there is a major third interval. The interval from B to D is a minor third, however note that only the intervals from the root to the other notes determine a chord’s quality.†

When someone says “play a G chord” they’re referring to a G major triad by default. When someone says “play a G minor chord” they’re referring to the G minor triad. There are other chords other than triads which are based on G, but if someone refers to a chord, they mean a triad unless otherwise indicated by additional words or numbers (other than major or minor).††

Seventh Chords

All seventh chords have four notes: a triad with a seventh. If we add the next third from the G major scale (F#) on top of our G triad, we have a G major 7 chord.  There are four different types of seventh chords: major 7, minor 7, diminished 7, and dominant. I won’t go in depth on these chords for now, but know that if you keep building in thirds on top of a 7th chord, all additional notes are called tensions. Tensions are not considered functionally essential to a chord but are said to add color.

*Inversion means you flip the interval. So a minor 2nd inverts to a major 7th, a perfect fifth inverts to a perfect fourth, etc.

**Only known as the leading tone in the context of a scale or key signature, not in the context of chords. So the major 7th on the chord would not be called a leading tone unless it were the Imaj7 chord.

***If you have ever looked at a jazz chart or tried to learn songs out of a guitar magazine, you’ve probably also seen 10ths, 11ths, etc. These are called tensions. They’re all based on the 13 basic intervals but have additional octaves in between them.

****There are also suspended triads, which use a major 2nd or a perfect fourth to take the place of a third.

†But if you really want to get crazy when thinking about chords, think of the relationship between every note in a chord and how this might subtly effect its impact on a listener. The spacing between the notes of a chord (the order in which you place notes, the octave registers you put them in, and the number of instances of any given note in a chord) is known as the chord’s “voicing.” Certain styles use certain types of voicings, and most instruments can play certain types of voicings more easily than others.

††For example, suspended (sus), augmented (aug), add 9, major 7 (“maj 7”), etc.