Ptolemaea is written in the key of D♯ Locrian. According to the Theorytab database, it is the 5th most popular key among Locrian keys and the 77th most popular among all keys. The D♯ Locrian scale is similar to the D♯ Minor scale except that both the 2nd and the 5th notes (E and A) are a half step lower. This means that the characteristic "home base" chord of D♯ Locrian has a dissonant diminished quality (d#˚), which makes it a very uncommon choice for songs. See the D♯ Locrian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!