What's a Tetrad ?
A basic chord is built with three notes this is what we call "triad chords" or "triads". Tetrad chords aka "four note-chords" are simply chords containing four notes, “tetra“ is a Greek root meaning four. This means that tetrads have an additional note, a major seventh or a minor seventh. In other words , a tetrad is a triad with a supplementary note. This article will enlighten you on how these tetrad chords are built and how they can be classified.
One of the fundamental theoretical elements to understand music is the harmonization of the major scale. Harmonizing scale is building chords with notes. For this, you have to stack thirds (It is also possible to harmonize the major scale in fourths).
If you are wondering why thirds and not seconds or sixths for example, the reason is mainly historical: our music today is based on harmony in thirds. Once you have read this lesson, you will be able to find the tonality of a song simply by looking at its chords, you will know which scale to play on which chord progressions.