Scales notes are used to build melodies, but they are also used to build the chords that we use to harmonize melodies. We can build chords over every degree of a scale:
M = major; m = minor; d = diminished
In all major scales, triads that are formed on degrees I, IV, and V are major. Those formed on degrees II, III, and VI are minor; the triad formed on degree VII is diminished.