An elementary approach to the fine structure of L

We present here an approach to the fine structure of L based solely on elementary model—theoretic ideas, and illustrate its use in a proof of Global Square in L. We thereby avoid the Lévy hierarchy of formulas and the subtleties of master codes and projecta, introduced by Jensen [1972] in the original form of the theory. Our theory could appropriately be called “Hyperfine Structure Theory”, as we make use of a hierarchy of structures and hull operations which refines the traditional Lα– or Jα–sequences with their Σn-hull operations.