I haven't had a chance to start it yet, but Christopher Burke's Paul Renner is likely to answer that question.

Renner himself in his 1922 book "Typografie als Kunst." Before he did the first drawing for what would become Futura he said that all of European writing is based on Roman versals, that were themselves defined by the most basic and distinct of graphic forms; the circle, the square, and the triangle. Covered in my thesis. http://typophile.com/node/18165