divided sharps

The term "divided sharps" refers to a special arrangement on keyboard instruments, primarily used in the 16th to 18th centuries with the meantone tuning system. This tuning provided purer sound for a limited number of keys but restricted the use of others. For instance, a key tuned to E♭ could not also function as D♯ without retuning.1

To address this, keyboards were built with divided keys. The front half of a key would sound the sharp note, while the back half sounded the equivalent flat. Commonly divided keys included A♭/G♯, E♭/D♯, and B♭/A♯. Some complex keyboards from the 19th century featured over 50 keys per octave, and others were designed to divide the octave into 24 parts for quarter-tone music. A "double sharp" in musical notation, however, is a character that raises a note's pitch by a whole step.12