The use of chords extensions in classical music is different from that in pop & jazz. In the classical notation, extensions are dependent on the key. Thus, C7 could mean a chord with a major seventh, if it is in the key of C major. Thus the seventh is indicating the seventh tone in the scale, counting from the note c. In the key of F major however, C7 will denotate a chord with a minor seventh, because the note b flat is part of the scale.

In pop and jazz however extensions are independent of the key. In those styles, a 7 will always indicate a chord with a minor seventh. A major seventh is always denoted by maj7 or ^ or something similar, but never by 7.

Musicians should therefore be aware of each other’s theoretical background. Unfortunately, the differences in notation between classical music and pop and rock are too large to ever hope for a unification in the chord language. These differences become even larger when chords are analyzed in roman numerals (also called scale degrees). We’ll get to that subject in a later post.