Trilogues are informal tripartite meetings attended by representatives of the European Parliament, the Council and the Commission during the co-decision process. Owing to the ad-hoc nature of such contacts, no "standard" format of representation has been laid down. The level and range of attendance, the content and the purpose of trilogues may vary from very technical discussions (involving staff level of the three administrations) to very political discussions (involving Ministers and Commissioners). They may address issues of planning and timetable or go into detail on any particular substantial issue. However, as a general rule, they involve the rapporteur (accompanied where necessary by shadow rapporteurs from other political groups), the chairperson of COREPER I or the relevant Council working party assisted by the General Secretariat of the Council and representatives of the Commission (usually the expert in charge of the dossier and his or her direct superior assisted by the Commission's Secretariat-General and Legal Service).
The purpose of these contacts is to get agreement on a package of amendments acceptable to the Council and the European Parliament. The Commission's endorsement is particularly important, in view of the fact that, if it opposes an amendment which the European Parliament wants to adopt, the Council will have to act unanimously to accept that amendment. Any agreement in trilogues is informal and "ad referendum" and will have to be approved by the formal procedures applicable within each of the three institutions.