Since we are starting in the middle of the piece, I will reveal the key signature: it is in C# minor.




The first half of measure 49 contains G#--B#--D#--F#, a G# dominant seventh chord. Since the B# is the lowest note, the chord is in first inversion.




The second half of measure 49 contains C#--E--G#, a C# minor triad.




The first half of measure 50 contains D--F#--A, a D major triad in first inversion. This is the Neapolitan sixth chord in C# minor.




The second half of measure 50 contains G#--B#--F#, a G# dominant seventh chord with a missing fifth.




The first part of measure 51 contains C#--E--G#, another C# minor triad.




Play these measures of Moonlight Sonata.