Coil Binders — Punching for Plastic Coil

Some had turned to oval holes. Oval holes were, for many years, the accepted way to battle thicker books with spiral wire and, even for a time, plastic coil. The old standard was a double "D" die. Why is it called double "D"? Imagine two Ds mirrored against each other and turned counter-clockwise 90 degrees. It is actually a round hole with its sides sliced off. A double "D" die used to be 9⁄64˝ (3.5mm) wide and 11⁄64˝ (4.365mm) tall.