Why is the sheet written this way (Moonlight Sonata 1st movement)?

[salim]

Moonlight Sonata 1st movement, the key signature shows to have C, D, F and G as sharps. In a few measures, there is a note for D with a sharp next to it.. it's played as D#, but shouldn't it be an E (since it has an accidental and the key signature with a sharp)? There are no oter Ds in the same measure that have the accidental cancelled so I'm a little confused..

[dcstudio]

no--it's a courtesy accidental-- because in mm 13-15 the D is natural--it's played as D#

[michael_c]

Accidentals are absolute, not relative. If you see a sharp in front of a D, it means D-sharp, no matter what the key signature is. Often an accidental is added as a reminder, as is the case here: as dcstudio says, it's a "courtesy accidental".

[salim]

Ahh ok, thanks!