Why did some 16th- and 17th-century keyboard instruments have divided sharp keys?

In the 16th and 17th centuries, some keyboard instruments featured divided sharp keys to accommodate the limitations of meantone temperament, the prevailing tuning system of the era.1

Unlike modern equal temperament, where enharmonic equivalents (such as G♯ and A♭) are represented by a single key, meantone tuning produced purer intervals for a limited number of tonalities at the cost of others. In this system, a single raised key could only be tuned to one specific pitch; for example, a key tuned to E♭ could not function as D♯ without manual retuning.12

To address this, builders created keyboards with divided raised keys. The front half of the key would sound the sharp, while the back half sounded the corresponding flat, allowing performers to access both pitches without retuning the instrument. These divided keys—most commonly for A♭/G♯, E♭/D♯, and B♭/A♯—were especially prevalent in Italian instruments of the period.1