A Dictionary of Music and Musicians/Double Sharp
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DOUBLE SHARP raises a note by two semitones, and is denoted by a ×, probably an abbreviation of ♯♯. It is singular that the sign should be a less complicated one than that for the single sharp. On instruments of fixed intonation C× = D♮, E× = F♯, etc. The French call it double dièse, and the Germans doppel kreuz. The Germans call the notes eisis, fisis, gisis, etc.