When was the Guidonian hand introduced?
Guidonian hand
The hexachordal system also distinguished between B♭ (fa in the F hexachord, and known as "B molle" for 'soft B') and B♮ (mi in the G hexachord, and known as "B durum" for 'hard B').  Over time, the soft and hard variants of 'b' were depicted as a rounded '♭' and a squared-off '♮' which gradually developed into the modern flat and natural signs (or, in Northern Europe, into the letters 'b' and 'h').[6]