Prelude No. 12 in G# Minor by Frédéric Chopin

Measures
9 and 10	A repetition of measures 1 and 2.
11	An E minor chord, B minor's IV degree. B major is G# minor's relative major key. Modal interchange between B major and B minor explains the E minor chord. The chord helps us arrive at an augmented sixth chord in the next measure.
12	The chord is written as a G dominant chord (G-B-D-F) in the first two beats, but it is as an augmented sixth chord (G-B-D-E#) of 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.