I have recently come across the statement that the Oblique Penholder was invented for Spencerian Script and not Copperplate. As the Oblique Penholder was invented and patented in England in 1831 and Spencerian script didn't appear until 1840 the writer is clearly mistaken.
To achieve the shaded downstrokes, the nib must point at, or as close to the downstroke as possible, This opens the tines of the nib evenly creating perfect shading. The occasional horizontal stroke is produced by turning the paper. still creating shaded downstrokes.
Although this is possible with a straight penholder, in order to achieve the nib pointing at the downstroke the wrist has to be turned to an uncomfortable angle and the paper rotated anti-clockwise, Furthermore, this rotation of the paper means that you are writing virtually uphill, making assessment of the lettering unnecessarily difficult. On the contrary, with the aid of the Oblique Holder it's possible to write for a long time without stress,
These illustrations show the tines opening evenly on the angle of the downstrokes.