Cis-Dur
Its relative minor is B-flat minor. Its parallel minor, D-flat minor, is usually replaced by C-sharp minor, since D-flat minor features a B in its key signature, making it impractical to use. C-sharp major, its enharmonic, with seven sharps, has a similar problem. Therefore, D-flat major is often used as the parallel major for C-sharp minor. (The same enharmonic situation occurs with the keys of A-flat major and G-sharp minor).