Harmonized Scales: Using 3rds & 6ths to Spice Up Your Clawhammer Arrangements. Learn series of simple two-string fingering shapes, or double stops, that let you harmonize any note with a third or sixth interval. Guaranteed to make even simple melodies sound gangbusters!! FYI, thirds are the building blocks of chords and sixths are inverted thirds (turned upside down). So in the key of C Major, for example, a series of thirds is C-E, D-F, E-G . . .; while the corresponding series of sixths is E-C, F-D, G-E.