Modus Tenendi Parliamentum
The Modus Tenendi Parliamentum (Method of Holding Parliaments) is a 14th-century document that outlined an idealised version of English parliamentary procedure. Part of its significance lies in its very title: parliament was now "seen as both institutionally well defined and a proper subject for description and conscious reflection".
Source: Wikipedia — Modus Tenendi Parliamentum (CC BY-SA 4.0)