Why did some early keyboards have both B and B-flat as natural keys?

Early keyboard instruments sometimes featured both B and B-flat as "natural" keys because the arrangement of keys was not yet standardized and depended heavily on the music being played and the prevailing musical theory of the time.1

During the medieval and Renaissance periods, musical theory was organized around the hexachord system. This system utilized a gamut of notes that included both B-natural and B-flat to accommodate different hexachords—the "hard" hexachord, which required B-natural, and the "soft" hexachord, which required B-flat. Because these notes were essential for singers to orient themselves and perform correctly within the modal system, instrument makers included both as primary keys.21

As musical styles evolved and the modern major-minor key system replaced the older modes, the need for such specific key arrangements diminished, eventually leading to the standardized keyboard layout familiar today.13