FreeTDS is an open-source C library that implements the Tabular Data Stream (TDS) protocol, providing client connectivity to Sybase and Microsoft SQL Server databases. It exposes three API layers: DB-Library (compatible with Sybase/Microsoft's db-lib), CT-Library (Sybase's client library), and a full ODBC driver. The library also includes a connection pool server, server-side test infrastructure, and command-line tools such as `tsql`, `fisql`, `freebcp`, `bsqldb`, and `defncopy`. Core protocol handling lives in `src/tds/`, which manages login, authentication (NTLM, Kerberos, SSPI), encryption (TLS via OpenSSL or GnuTLS), packet framing, query submission, cursor management, and bulk copy. The architecture is layered and modular, with clean stream I/O, threading, and character conversion abstractions that make components independently reusable. The codebase is cross-platform (Unix, Windows, VMS) and supports TDS versions 4.2 through 8.0, adapting to server-specific behaviors. Despite its age and some legacy compatibility code, it remains actively maintained and widely used as a reliable database connectivity layer in production environments.