Solid (derived from "social linked data") is a proposed set of conventions and tools for building decentralized social applications based on Linked Data principles. Solid is modular and extensible. It relies as much as possible on existing W3C standards and protocols. Solid uses WebID URIs as universal usernames or actor identifiers. Frequently referred to simply as WebIDs, these URIs form the basis of most other Solid-related technologies, such as authentication, authorization, access control, user profiles, discovery of user preferences and server capabilities, and more.