String-of-leaves ~ scrolls ~ concertinas ~ codices. For many years I associated codex with ancient ~ written ~ archaeological finds. I was not wrong over that, but I had completely missed the point that codices was a descriptor of the binding technique. Certainly from the selection of bindings at the start of this paragraph ~ all of which are no longer used ~ it is the most now popular binding method ~ and it has been for centuries.
A folio is a piece of paper ~ or similar material ~ folded into half. When a folio is sewn to other folios ~ either tucked inside ~ or adjacent ~ or both ~ then that is codex binding ~ or codex sewing. Historians may wish to reserve the name for handwritten early books ~ as in paragraph one. It will be obvious to readers by now that I do not subscribe to that narrow definition.
Without the fold for the thread to link the pages together they have have to be joined somehow ~ and in order ~ to make a useful book. Other alternatives are stab sewing [117565] or edge gluing [115742]. Other binding names ~ such as Coptic or Longstitch ~ are also in the codex binding family.