I mean — where does this convention come from?
Screws themselves have either a #numeric value or a fractional value. This refers to shaft of the screw. But what of the head and the tools used on them?
According to Fastener Engineer, Lee Dougan of the Phillips Screw Company:
. . . The original Phillips Screw driver bit goes back to the 1930’s. But prior to the invention of the Phillips® cruciform drive system, Robertson invented the Robertson® Square drive which was a marked improvement over the conventional slotted drive system that had been around for centuries. These drive systems were designated #0, #1, #2, and #3.
A #2 Phillips driver bit is approximately the same size that would fit into the equivalent screw head that would use a # 2 Square. Similarly, the #1 Phillips and #1 Square fit into similarly sized screw heads. So I think that is where the Phillips Screw designation came from. (It) takes you back to the Robertson square designation. A simple numbering system is an easy way to distinguish similar looking parts. It may be as simple as that . . . .
Note from the Host:
Please read below in the Comments section for the complete journey to our best answer above. I welcome continued discussion on this. A follow up post of sorts, here >> The Skinny on Screws :: A Guide to Common Screws.
Screws are one of the wonderous Six Simple Machines. More on all of them there. Thanks. ~jb @BuildingMoxie