How to interpret letter and roman numeral symbols for chords.
First off, a quick description of what a chord is: A chord is a combination of 3 or more notes. The most common types of chords are triads, which are 3 notes each a third apart from each other (one note in between–C and E form a third with D in between). So for example, a C major triad would be the notes C E G.
The other common type of chord are seventh chords, which are triads with another note (‘the seventh’) a third above the top of the triad. A C major seventh chord would be C E G B.
There are two main ways of writing chords shorthand: Letters or Roman Numerals.
How to Read Letter Notation:
The letter determines the key of the chord and is generally the bottom note of the chord. It is what the other notes of the chord are based on.
Triads: By default triads are major (abbreviated Maj or M), but chords can also be minor (abbreviated min or m), which has a lowered/minor third, diminished (abbreviated dim or °), which has a lowered 3rd and 5th, or augmented (abbreviated aug or +), which has a major 3rd and a raised 5th.
Dominant 7th - the default seventh chord, with no abbreviation - just 7. A dominant 7th is a minor third above a major triad (same as a minor seventh over the root)
Major 7th - abbreviated Maj7, M7, or Δ7. A major 7th is a major third above a major triad (a major 7th above the root)
Minor 7th - abbreviated min7, m7, or -7. A minor 7th is a minor 3rd above a minor triad (a minor 7th above the root)
Half-diminished 7th - abbreviated ø7. A half-diminished 7th is a major 3rd above a diminished triad (a minor 7th above the root)
Fully-diminished 7th - abbreviated o7. A fully-diminished 7th is a minor 3rd above a diminished triad (a double flat 7th above the root)
Inversions: Inversions change which note is the bottom of the chord. A first inversion chord has the 3rd of the chord on the bottom; A second inversion chord has the 5th of the chord on the bottom, and so on. Here’s a table guide to inversion abbreviations:
Alterations: Alterations are the most straight-forward part of reading a chord symbol. An alteration is a little note in the superscript of a chord that essentially marks an accidental in the chord. For example, a Cø7 chord could also be written with an alteration as Cm7 (b5)
How to Read Roman Numeral Notation:
Roman Numerals are very similar to Letter Notation - the markings for seventh chords and inversions are exactly the same, the difference is in how the key and “quality” (major/minor) of the chord is represented.
Key: the key/root of a chord is conveyed by a roman numeral from I to VII, where I is the first note of key of the whole piece, II is the second note, and so on. So in C major, a V chord would have G as its root note (because G is the 5th note in a C major scale)
Quality: the quality of a chord is shown by the capitalization of the roman numerals. Capital numerals (I, VI, etc.) are used for major or augmented chords and lowercase numerals (i, vii) are used for minor or diminished chords.