Chord progressions

If the third is minor (1–m3–P5), we say the triad is minor and write it with a lowercase roman numeral, where the number is the position of the root note (1) in the sequence of notes in the scale. If the third is major (1–M3–P5) then it’s called major, and we write it with an uppercase roman numeral. And if the fifth is diminished (1–m3–d5), we call it minor diminished (it cannot be a major diminished chord, because there is only one tone between the major third and the diminished fifth!). For example, if we are playing with the scale of white keys with the root note being C, and we play a triad on the third note E, we play E–G–B, which is E minor written 'iii'.