 It has turned out subsequently that the axiom V = L gives a virtually complete extension of ZFC. This means that, apart from sentences arising from Gödel’s incompleteness theorems, essentially all set-theoretical questions can be decided by means of the axioms V = L. This is not to imply that such results are in any way trivial. Indeed, it has turned out that L is quite a complicated structure, despite its relatively simple description. As for settling open set-theoretical questions in L, the main step was the emergence of Jensen’s fine structure theory of L (Jensen 1972). Recalling that the successor step Lα +1 in the definition of the constructible hierarchy adds to L all subsets of Lα  definable by first order formulas φ over (Lα, ∈), fine structure theory, roughly speaking, ramifies the step from Lα  to Lα+1 into smaller steps according to the complexity of the defining formula φ. Jensen established by means of his fine structure a strengthening, denoted by ◊, of CH, that he used to construct a Souslin tree in L, and a combinatorial principle □ that he used to show that the Souslin Hypothesis is consistent with CH.