Music theory is quite mathematical, but one of our music theory specialists here at 24HourAnswers.com is quite human - always ready to help students with whatever they need. This is an article he wrote recently on chords:
As I sit in front of my piano, pondering what I want to play next, I doodle around in the key of C and start to play some chords. The C major triad takes me back to my early days as a musician, when I was just figuring out harmony and melody; but as I add the seventh my thoughts change to what the weather will be like today. I bet you're wondering what made my thoughts shift so dramatically? Well, a major seventh chord has a completely different feeling then a major triad, to me anyway, but I'm getting ahead of myself.
Chords come in all shapes and sizes and the most basic of those is a major triad. A triad is a group of three notes played together and a major triad is formed by playing the first, third, and fifth scale degrees in a major scale. Major triads are the most stable, as far as chords go, and tend to be bright or happy sounding. If I take the third scale degree and flat it (lower it by one half step), then I have made a minor triad. Minor triads sound slightly darker and sadder then their major triad cousins. The last type of triad chord is called a diminished triad and it is formed by flatting the third and fifth scale degrees. These chords have a tendency to sound suspenseful and are great for adding tension to a piece. Triads, however, are not the only types of chords we musicians have at our disposal.
Let's revisit the added seventh that led to my earlier abrupt change of thought. I began by playing a C major triad and then I added the seventh scale degree to the chord. This formed a C major seventh chord and led to my thoughts on the weather. Whenever I turn to the weather channel on the TV they are always playing some type of smooth jazz. Smooth jazz is notorious for its use of major seventh chords which gives it that relaxed, easygoing kind of feel. It also leads us to the world of seventh chords, a tool that composers have used for centuries to add variety and interest to their music.
When we take a major seventh chord and flat the seventh scale degree a major minor seventh chord is the result. Most musicians don't refer to this chord as a major minor seventh, however, because the name is just too long. Most call it either a dominant seventh chord or just a plain seventh chord. The term dominant seventh comes from the common progression of chords in western music, but that's a subject for a different article. Jazz musicians have used seventh chords for the past hundred years or so making the chord synonomus with the genre.
As we move on through our chordal journey we come to the minor seventh chord. This chord is formed by flatting both the seventh and the third scale degrees. This chord is also used heavily in jazz music and the addition of the flat seven to the minor triad makes this chord sound less sad then the minor triad alone, while also providing natural motion to the chord progression. All seventh chords allow for a feeling of movement and expectancy because they are naturally less stable then their triad counterparts. This is due to the addition of the seventh scale degree which makes the chord top heavy. The general rule of thumb is that the less harmony the more stable the chord and by adding the seventh scale degree we are in essence adding extra harmony.
Our final two chords are known as half diminished and fully diminished chords. A half diminished chord is formed by adding a flat seventh to a diminished triad while a fully diminished chord is formed by adding a double flatted seventh. A double flatted seventh is a seventh that is lowered by a whole step as opposed to a half step. Both of these chords are very unstable due to the addition of the seventh scale degree and the natural dissonance of the chords. These types of chords have been used since the time of Bach to add suspense and dread to compositions.
As I go back to my piano to write some music, I keep all of these chords in mind. Using the different harmonies and tendencies of each of these chords opens up a world of musical combinations. I could take a minor triad, follow it up with a dominant seventh chord, and finish with a major triad to form a neat little cadence to finish off a phrase of music. Music has its own set of rules, and learning them all (and when to break them!) can be a challenge. Scales, keys, chords, rhythms, notation, articulations, harmonic theories, compositional practices—yes, it’s a lot, and yes, an understanding of basic music theory is absolutely critical. After all, if you can’t read music, you can’t study it. Fortunately, there are plenty of helpful websites that offer tutorials in every aspect of music theory. Ricci Adams' musictheory.net provides excellent overviews and helpful exercises in basic music theory; the site teoría offers articles and analysis in addition to lessons and exercises. The Society for Music Theory provides many useful resources as well, including access to Music Theory Online, an academic journal for music theory.