Wine (Wine Is Not an Emulator) is a compatibility layer that enables running Microsoft Windows applications (DOS, Win16, Win32, Win64) on Unix-like operating systems. It reimplements Windows API DLLs, kernel drivers, system services, and command-line tools using open-source code, supplemented by bundled third-party libraries (e.g., musl, libtomcrypt, vkd3d, capstone, fluidsynth). The architecture is a loader that launches Windows executables and delegates system calls to Unix-side code via a well-defined unixlib bridge, with hundreds of DLLs providing COM, DirectX, GDI, networking, audio, and other subsystems. The codebase is heavily modular, with each DLL in its own subdirectory, extensive test suites using the Wine test framework, and a custom build system (autotools with code generation tools like widl, wrc, wmc). While functionally extensive, many DLLs contain stub or incomplete implementations, and bundled third-party libraries pose version drift risks. The project is actively developed (frequent commits, recent additions from 2024-2025) and demonstrates strong engineering patterns for cross-platform Windows API reimplementation.