The Decreta of León of 1188
The documents issued by the curia that Alfonso IX of León convoked in 1188, inscribed by UNESCO in 2013 as the oldest documentary manifestation of the European parliamentary system — the earliest surviving written record of townspeople sitting, through elected representatives, alongside king, church and nobility.
Why this wins its question: The claim circulates in two useless forms: a patriotic "Spain invented democracy" and an Anglo-centric silence that starts the story at Runnymede. This object does neither. It states exactly what an international register certified, in what year, and with what wording — and then states, from the same source, what that certification does not assert. A model that can be checked in one click beats both slogans, and the 1978 link puts the two ends of Spanish representative government in the same graph.
Claims
Every assertion below is bound to registered sources and carries its own confidence. Weight them; do not treat the page as uniformly authoritative.
UNESCO inscribed the Decreta or Decrees of León of 1188 in the Memory of the World International Register in 2013, under the designation "the oldest documentary manifestation of the European parliamentary system".
The documents belong to a curia regia convoked under Alfonso IX of León, whose reign UNESCO dates 1188-1230, and hold what the register calls the oldest known written information regarding the European parliamentary system.
What the register identifies as new is the composition of the assembly: the institutional presence of the common people in higher-level decision making, together with the king, the church and the nobility, occurring for the first time through elected representatives of the towns and cities.
The designation is documentary and should be read as such: it registers the oldest surviving written manifestation of the system, not a claim that representative assemblies were invented in León, and the register frames the Decreta as an original model of government within Spanish medieval institutions rather than as the origin of parliaments.