Decentralized social-networking protocol ('at proto') that Bluesky is built on, but which also supports many other apps. Defines records (JSON documents grouped in collections inside user repositories held by a PDS), lexicons (JSON-Schema-like collection definitions, e.g. app.bsky.feed.post), at:// URIs of the form at://<did>/<collection>/<rkey>, DIDs (did:plc or did:web) as permanent identifiers, and handles (domains) verified via DNS TXT or a well-known HTTPS file. Network roles: PDS (source of truth per account), relay (aggregates PDS websocket firehoses), App View (materialized view that counts likes, assembles threads, etc.), and client apps. Data is binary-encoded as CBOR on websockets, repositories exportable as CAR files for account migration. The permissionless design lets anyone build Twitter-, Instagram-, GitHub- or blog-style apps and interoperate purely via shared lexicons.