The theory of constructible sets owes much to the work of Ronald Jensen. He developed the so-called fine structure theory of \(L\) and isolated some combinatorial principles, such as the diamond (\(\diamondsuit\)) and square (\(\Box\)), which can be used to carry out complicated constructions of uncountable mathematical objects. Fine structure theory plays also an important role in the analysis of bigger \(L\)-like models, such as \(L(\mathbb{R})\) or the inner models for large cardinals (see  Section 10.1).