France and Germany are holding their first joint parliamentary session as part of their commitment to forging closer ties.
The 50 deputies from each country are from almost all political parties, proportional to their numbers in the national parliaments.
The plan is to convene the new joint parliament twice a year in the future.
It follows the signing of the Treaty of Aachen, which aims to bolster Franco-German co-operation in Europe.
The new parliament's purpose is to enforce that treaty, signed in January, and to monitor "affairs of common interest" – including foreign policy, security and defence.
The joint parliament will not be legally binding on the national parliaments of either country though.
Monday's first session is mostly ceremonial and procedural, with the signing the agreement for the joint venture by parliamentary presidents Wolfgang Schäuble of the German Bundestag and Richard Ferrand of the French National Assembly.
Mr Schäuble said the new organisation "underlines our commitment to place bilateral cooperation in a European context."
"Germany and France have a special responsibility for Europe," he added.
Mr Ferrand tweeted a photo of the signing, calling it a "historic day".
"If the Franco-German engine did not work, the whole of Europe would slow down," he told the assembled politicians.
#DirectAN Aujourd’hui siège à l’@AssembleeNat la 1ère Assemblée franco-allemande #APFA ! En ce jour historique, j’accueille Wolfgang Schäuble, qui la co-préside à mes côtés.— Richard Ferrand (@RichardFerrand) March 25, 2019
Au programme: signature de l’accord🇫🇷🇩🇪, élection du bureau, allocution des groupes politiques, audition✅ pic.twitter.com/q7CCiLoucz
Over time, there are plans for this joint parliament to be part of strengthening links between the national parliaments of both countries.
What is the treaty about?
The Treaty of Aachen was signed by French President Emmanuel Macron and German Chancellor Angela Merkel earlier this year.
At a time when one member of the European Union is leaving, and amid the rise of populism or right-wing politics in several member states, the pair spoke about the need to strengthen European values.
As well as common diplomatic goals, the two nations committed to joint defence and "a common military culture".
It commits both countries to agreeing a common position - and to issue joint statements - on major EU issues, rather than pursuing separate agendas. They will do the same at the United Nations.
But Italy's interior minister has been a vocal opponent of what he calls the "Franco-German axis", and is widely expected to spearhead a Eurosceptic alliance in May's European elections.
Even European Commission President Jean-Claude Juncker issued a warning, at the signing of the treaty, that "strengthened co-operation in small formats is not an alternative to the co-operation of all of Europe."