Pianists, guitarists, and anyone else with an instrument that can sound more than one note learns early to think in chords. Chords make up most of our concept of harmony, but what is a chord?
Chords and Harmony
The concept of harmony has evolved in Western music over time. At one point, only unison (the octave) was considered harmony. Later, the perfect fifth was added, and finally, more notes were added to modern harmony.
A chord is built on the concept of harmony. Some notes sound excellent together, but others sound hideous. In general, a chord is a grouping of a few notes that sound (more or less) harmonious.
Building a Chord
Building a chord is as simple as playing a few harmonious notes together. To experiment, on a guitar or piano, play a C and a G together. These two notes represent a fifth, generally considered the most harmonious of relationships. Any note and the note seven semitones above it are a perfect fifth. This is the basic “power chord” of electric guitarists, used because it is neither major nor minor, and because it creates interesting patterns in distortion.
Throwing an E in the mix makes it into a C major chord. It is called C because C is the lowest note in the scale that is included. You can “invert” the chord by playing the C in a higher octave than the G as well. It is major because the E is a major third (four semitones) above the C. A C minor chord is made by playing E-flat instead of E, which is a minor third (three semitones) higher. In general, major chords are perceived as “happier” while minor chords are “dramatic” or “sad.”
Adding to the Mix
There are as many chord designations as there are roots and combination built on them. For instance, you can include a minor-seventh to the major chord to make a major seventh chord. You can also throw on a seventh into a minor chord to make a minor seventh.
You can augment (add a semitone), diminish (steal a semitone) or twist around any member of a chord. As a rule, the more you tweak, the more you will run into discordant combinations. Typically two notes that are right next to each other on the scale, even in different octaves, will make less harmonious sounds. Composers use this trick to create tension in the music, but ending on one of these horrible chords is generally not done except for explicit artistic effect. Instead, the chord may transition to the nearest pretty chord, relieving the perceived tension.
Chords for Other Instruments
Many instruments can’t play chords, but need to know them anyway. When improvising music especially, the notes you pick from as you play typically are built around the chord implied within the music. Different musical styles imply different chords, such as jazz and blues players using different chords from classical players. As an instrumentalist, know the musical style you are playing in, and learn to play within (and skirt carefully) the norms and harmonic conventions before you try and show off.
MusicLearning.com: Guitar Chords and Diatonic Chord Construction
Saxstation.com: Active Ingredients for Chords on a Saxophone