When common chords are diagrammed using intervals, a logical ascending order begins to form. Notice how all the chords above are similar. They all contain a bass note (the top and bottom notes represented by the circle), and the note seven intervals away from the root note higher in pitch (indicated by the X). Only one note, the third note (represented by the triangle), changes position.
The chords above are considered triads because they contain exactly three notes. In research, only a few chords were found with more than four notes. And since these 5-tone chords had very muddy or unintelligible sounds and scales usually contain five notes or more, it became practical to think of chords as combinations of four notes or less played simultaneously. However, any combination of any number of notes played simultaneously should be considered a chord.