Burroughs' Encyclopaedia of Astounding Facts and Useful Information, 1889

As said before, it matters not so much what angle of slant is adopted in writing, provided it is made uniform, and all letters are required to conform exactly to the same slant. Writing which is nearest perpendicular is most legible, and hence is preferable for business purposes. The printed page of perpendicular type; how legible it is. But for ease in execution, writing should slant. It follows then that writing should be made as perpendicular as is consistent with ease of execution. The slant of writing should not be less than sixty degrees from the horizontal.