Why do we only have Major and Minor keys?

So why do we only have Major and Minor keys?

What about diminished and augmented keys?

evo

Hi; im not sure as whether there are musical pieces written in keys other than either major or minor; theoretically perhaps, it could be thought of a piece whith starting and ending chord being a diminished (or augmented) chord. It may be the case that there are so many (indeed, infinite) different ways to develope a piece starting either in a major or minor chord, that historically those keys had attracted the main effort by composers and music artists of all time, so to speak. Greetings

It's mostly historical. Gregorian Chant was written without much thought given to keys or the like; it was melodic only, but there were traditional composition procedures. Later, people tried to classify chant as to its characteristics; some chants were clearly related in style and form. This procedure gave rise to eight modes comprising four ending notes (D,E,F,G) and ranges basically D-D or A-A and E-E and B-B and F-F and C-C and G-G and D-D. Amusingly the note Bb was in existence by the time this classification was made. The F mode sometimes had a Bb which made it equivalent to a (hypothetical) mode on C.

As music underwent changes, more modes were suggested and pieces could have parts written in other than their beginning modes; this led to the addition of more notes like Bb (Eb to start with, etc.) In parallel with this was a 6-note pattern called the Hexachord system (it's worth reading the Wiki on hexachords. It's complicated and more so when combined with the chant modes,

During the 1300-1500 time frame, harmonic concepts were expanded into the current scheme used now, but not with the same usages. The chords (triads anyway) were known and used but a good description wasn't available. The musicians (as usual) outran the theorists.

By the 1600s, it was clear the that modes on D and E (as then used) were characterized by the having a minor third (D to F and E to G respectively); the modes on F and G were characterized by having a major third (F-A and G-B) similarly placed. It was recognized that modes on A and C were also becoming common.

Because of the above characterization of the modes, those with a minor third at the beginning (A,D,E) were termed minor and those starting with a major third (A,F,G) were called major.

Composers also found that the distinction between major and minor was greater that that between other possible modes.