Therefore – given that a particular melody or arrangement of note pitches can be written either as D Flat Major or C Sharp Major and will be played on the same keys or frets either way (and sound the same), most composers will choose D Flat because instead of seven incidentals (7 sharps) to deal with in the key signature for C Sharp Major, there are only five incidentals (5 flats) in the key of D Flat Major. So it may be helpful to understand that on any keyboard instrument (such as piano) or fretted instrument (such as guitar), the notes played in the D-Flat Major Scale will be exactly the same as the notes in the C-Sharp Major scale. But the exact same black key can also be identified as a D flat – and will be referred to as such in pretty much any key with flats in the key signature (with occasional exceptions).

The black key to the right of any C key on a piano keyboard is in fact a 'C-sharp' and will be most likely be referred to as such when playing a piece in any key that contains a C sharp in the scale or in any key with sharps in the key signature for that matter. What that means is that all of the notes in the C sharp major scale sound pretty much exactly the same (to the human ear) as all the notes in the D Flat major scale – only they are notated (written) differently.

But the key of C Sharp Major is the “enharmonic equivalent” of the key of D Flat Major.

Yes indeed there is a key of C Sharp Major (C# Major).

B major scale d flat