Is there such a thing as a flat15 chord?

Yes, you could call it G9♭15, but it would be a pointlessly irregular usage of the chord naming conventions that music theorists (mostly) agree upon.

1. Are there 15th chords?
2. What is an extended chord music?
3. How do you extend a chord?

Are there 15th chords?

a 15th chord is a chord that adds the diminished or augmented 15th, together with the root and the 9th. however, if either of these 2 would not be presented, the 15th would have taken the function of the missing note. An example of a full 15th chord is C(root)-D(9th)-G-E-A-F#-C#(15th).

What is an extended chord music?

In music, extended chords are certain chords (built from thirds) or triads with notes extended, or added, beyond the seventh. ... Chords extended beyond the seventh are rarely seen in the Baroque era, and are used more frequently in the Classical era. The Romantic era saw greatly increased use of extended harmony.

How do you extend a chord?

Extended chords are simply chords that have notes which extend further than the standard three note triad. They're formed by stacking thirds on top of the base triad. Extended chords provide another layer of sound above general major and minor triads.