What are "Symons forms"?

You hear the term "Symons forms" all the time, but what forms do people really mean?
Most of the time, contractors are referring to the steel-framed and plywood-faced Steel-Ply, which is manufactured by Symons.

Symons makes many other concrete forms, so why do builders call Steel-Ply "Symons forms"?
Steel-Ply has been around for over 50 years, they are Number 1, and they are everywhere, so it is only natural for them to be called "Symons forms".

Why are they so popular?
Instead of contractors having to build forms out of plywood and lumber and drilling holes for ties and for bolting them to each other the way generations of concrete contractors had to do before Steel-Ply, they can buy a bunch of standard forms that are reliably strong and easy to connect up. In the building boom that was going on in the 1950's when Steel-Ply was introduced, and in all the ones since, contractors snapped it up. It stays popular because it's a great form.