Prelude No. 12 in G# Minor by Frédéric Chopin

Measures
9 and 10	Repeat measures 1 and 2.
11	An E minor chord is the IV degree of B minor. B major is the relative major key of G♯ minor. The modal interchange between B major and B minor explains the E minor chord. This chord leads to an augmented sixth chord in the next measure.
12	The chord is written as a G dominant chord (G-B-D-F) for the first two beats, but is actually an augmented sixth chord (G-B-D-E#) in the key of B major.
13 to 17	Cadence to B major.
17 to 20	Constant use of E minor, B minor's IV degree.