Beneath NixOS, Nixpkgs, and the Nix expression language is the “Nix store” interface — a deceptively powerful interface that includes includes both storage itself and also building. Although we rarely use it directly, it’s at the heart of what makes Nix great. By understanding the semantics of the Nix store interface in isolation, intermediate and advanced Nix users can expand their intuition and problem solving skills while working with Nix. The ways we both use and implement the interface will keep evolving in myriad ways, but the interface itself, or at least the core ideas behind it, have remained remarkably stable, anchoring everything else we do.