The idea started with the 1934 sheet music of “Mazurka Amabile”.  Its hand drawn title had most of the letters rendered in a rectangular shape [‘square’ in the sign trade] that featured rounded corners and terminals made by the shape of the lettering pen nib.

A few letters were rounder in design than others, so those were scrapped in favor of a more consistent character shape throughout the font. 

Pen Nib Square JNL is available in both regular and oblique versions.