An exception to this are so-called passerelle clauses in which the legislative procedure used for a certain policy area can be changed without formally amending the treaties.

[3] The precise nature of this organisation has been discussed extensively in academic literature, with some categorising the European Union as bicameral or tricameral,[4][5][6][7] though the European Union itself has not accepted such categorisation and it is generally considered to be sui generis by observers, given the unique dynamics between the legislative bodies not found in traditional tricameralism.

[citation needed] Under the ordinary legislative procedure (see below), the negative opinion from the Commission also forces the Council to vote by unanimity rather than majority[11] except when a conciliation committee has been set up.

It organises itself as a normal multi-party parliament in conducting most of its work in its committees and sitting in political groupings rather than national delegations.

Taken into account that the Commission (and also the Council) are the antagonists of the parliament in the system of separation of powers this is considered a democratic deficit.

The national parliaments of EU member states have an "early warning mechanism" whereby if one third raise an objection – a "yellow card" – on the basis that the principle of subsidiarity has been violated, then the proposal must be reviewed.

If the logistical problems of putting this into practice are overcome, then the power of the national parliaments could be decried as an extra legislature, without a common debate or physical location: dubbed by EU Observer a "virtual third chamber".

At the second reading, the act is adopted if Parliament approves the Council's text or fails to take a decision.

It was renamed the ordinary legislative procedure and extended to nearly all areas such as agriculture, fisheries, transport, structural funds, the entire budget and the former third pillar by the Treaty of Lisbon.

The trilogue negotiations aim at bringing the three institutions to an agreement, to fast-track the ordinary legislative procedure.

The expression "formal trilogue" is sometimes used to describe meetings of the Conciliation Committee,[24] which take place between the second and the third reading of a legislative proposal.

[25] The principal tool used in trilogues is the four column document, a working sheet divided in four sections, each of them comprising the positions of the three EU institutions.

[35] This applies to admission of members, methods of withdrawal, subsidiary general legal basis provision and combating discrimination.

[34] The domestic legal systems of Member States are mostly a legacy of different historical legislation[36] each of which has to be adapted in order to play an essential role in ensuring the standards of European Union law are implemented effectively, and uniformly.

Member States governments have a EU treaty obligation to amend their existing primary and secondary legislation in a way that is reasonably consistent and comprehensible to individuals and businesses in order to enforce EU legislation and directives consistently and reliably across all the various jurisdictions of each member state of the European Union in a timely manner.

In a few limited areas, the Commission has the authority to adopt regulatory or technical legislation without consulting or obtaining the consent of other bodies.

The 2009 Lisbon Treaty created two different ways for further amendments of the European Union treaties: an ordinary revision procedure which is broadly similar to the past revision process in that it involves convening an intergovernmental conference, and a simplified revision procedure whereby Part three of the Treaty on the Functioning of the European Union, which deals with internal policy and action of the Union, could be amended by a unanimous decision of the European Council, provided there is no change to the field of competence of the EU, and subject to ratification by all member states in the usual manner.

The Treaty also provides for the Passerelle Clause which allows the European Council to unanimously decide to replace unanimous voting in the Council of Ministers with qualified majority voting in specified areas with the previous consent of the European Parliament, and move from a special legislative procedure to the ordinary legislative procedure.

A decision has direct effect, but only relating to a specific person or entity, and there are also various other non-binding instruments.