From The Whole psalmes in foure partes, published by John Day . . . The admirably drawn frontispiece to the book shows us the father of a family instructing his household in singing. He is using the device of the ‘Guidonian Hand’ (dating from about A.D. 1030, and long taught to every choir-boy throughout Europe). This, the invention of Guido d’Arezzo (c. 995-c. 1050), made use of the hand as a sort of Music Map. The present Sol-fa syllables are a relic of the Guidonian hexachordal system. The particular gesture the artist has represented indicates, as near as we can put it today, ‘G = Doh’.

Percy A. Scholes, The Puritans and Music in England and New England: A Contribution to the Cultural History of Two Nations, 272.