Thispapergivesaformaldefinitionoftheprotocolspeci-fication language Scribble. In collaboration with industry, Scribble hasbeen developed as an engineering incarnation of the formalmultipartysession types. In its ten years of development, Scribble has been appliedand extended in manyfold ways as to verify and ensure correctness ofconcurrent and distributed systems, e.g. type checking, runtime monitor-ing, code generation, and synthesis. This paper introduces a core versionof Scribble,Featherweight Scribble. We define the semantics of Scrib-ble by translation to communicating automata and show a behavioural-preserving encoding of Scribble protocols to multiparty session type.