Tertian harmony is defined as chords built utilizing the intervals of thirds. Thirds are simply every other note of the major scale. When these thirds are stacked on top of each other, the largest chord that can be built is a 13th chord.
This approach utilizes all 7 notes of the scale (mode) with the following formula: 1, 3, 5, 7, 9, 11, 13. This resulting 7 note group is called an heptatonic arpeggio. The advantage of knowing your modes in terms of the heptatonic arpeggios is that you can clearly see all the harmonic options that are available. Depending upon the formula for the mode that you're dealing with, certain scale degrees in this general heptatonic formula may be altered with a flat or sharp.