What is a diatonic chord?

A diatonic chord is a chord that is made up from the notes of a key.  As a key contains seven notes, there are seven diatonic chords in each key.  A chord can be built on each of the seven degrees or notes of that key.  The diatonic chords in each major key are numbered with the Roman Numerals I, ii, iii, IV, V, iv, vii.  The reason why some numerals are capitalized and others are not, is that, the capitalized numerals are major chords and the lower case numerals are minor chords.  The exception is, vii, which is a minor 7 flat 5 chord.

  Progressions

A progression is a series of chords.  We can use these numerals to describe chord progressions across different keys.  I IV V is one such example.   The I IV V progression is the series of chords that is used in 12 barre blue.  It could go for example in the key of G, G major C major D major.  Or in the key of A, A major D major E major.  So we see that the intervals between, and with in, these chords are the same.