Yet as the subject expanded, its foundational literature grew vast, scattered across thousands of pages of dense monographs and seminars, much of it written in French. A great deal of the subject's working knowledge also lived as unwritten folklore passed down from advisor to student. Even defining an algebraic stack, a geometric space capable of remembering the internal symmetries of the objects it parametrizes, requires a formidable tower of prerequisites spanning commutative algebra, topologies, sheaves, and cohomology. Mathematicians needed a unified, self-contained reference where those prerequisites were written down in full detail.