1. A. Stubb, ‘A Categorization of Differentiated Integration’, 34 JCMS (1996) p. 283: G. Glöckler, ‘Flexibility and Closer Cooperation’, in G. Glöckler, et al., Guide to EU Policies (London, Blackstone Press 1998) p. 63, similar definition in B. de Witte, D. Hanf and E. Vos, eds., The Many Faces of Differentiation in EU Law (Antwerp, Intersentia 2001) (’The facilitation and accommodation of a degree of difference between Member States or regions in relation to what would otherwise be common Community or Union policies’).
2. See generally on differentiation: F. Tuytschaever, Differentiation in European Union Law (Oxford, Hart Publishing 1999); De Witte, Hanf and Vos, eds., op. cit. n. 1; G. de Búrca and J. Scott, eds., Constitutional Change in the EU (Oxford, Hart Publishing 2000); K. Inglis and A. Ott, The Constitution for Europe and an Enlarging Union: Unity in Diversity? (Groningen, Europa Law Publishing 2005).
3. Tuytschaever, op. cit. n. 2; De Búrca and Scott, op. cit. n. 2; De Witte, Hanf and Vos, op. cit. n. 1; J.A. Usher, ‘Variable Geometry or Concentric circles: Patterns for the EU’, 46 ICLQ (1997) p. 243; C.-D. Ehlermann, ‘Engere Zusammenarbeit nach dem Amsterdamer Vertrag: Ein neues Vertragsprinzip’, Europarecht (1997) p. 362 at pp. 366–367; S. Weatherill, ‘“If I’d Wanted You to Understand I Would Have Explained It Better”: What is the Purpose of the Provisions on Closer Cooperation Introduced by the Treaty of Amsterdam?’, in P. Twomey and D. O’Keeffe, eds., Legal Issues of the Amsterdam Treaty (Oxford, Hart Publishing 1999) p. 37 and J. Shaw, ‘Flexibility in a “Reorganized” and “Simplified” Treaty’, CMLRev. (2003) p. 279; C. Törö, ‘The Latest Example of Enhanced Cooperation in the Constitutional Treaty’, ELJ (2005) p. 641; D. Thym, ‘The Political Character of Supranational Differentiation’, EL Rev. (2006) p. 781; M. Kellerbauer, Von Maastricht nach Nizza, Neuformen differenzierter Integration in der Europäischen Union (Berlin, Duncker & Humblot 2003).
4. A. Arnull, A. Dashwood, M. Dougan, M. Ross, E. Spaventa and D. Wyatt, Wyatt and Dashwood’s European Union Law, 5th edn. (London, Sweet and Maxwell 2006) p. 112.
5. Structural differentiation results from the pillar structures, such as participatory differentiation which is characterised by transitional arrangements of Accession Treaties, opt-in and out-out arrangements and enhanced cooperation. Procedural differentiation is defined by the procedural variants within the Community model, such as for the Economic and Monetary Policy, for free movement of persons after Amsterdam and the procedure under Art. 67(2) EC. All of these categories highlight that from the legal perspective, it matters whether such differentiation is initiated at intergovernmental level, by means of an IGC, through an accession treaty, based on primary law provisions of EU or EC Treaty, or even on international agreements and secondary EC law. On these last examples, see further E. Vos, ‘Differentiation, Harmonisation and Governance’, in B. de Witte, D. Hanf and E. Vos, eds., The Many Faces of Differentiation in EU Law (Antwerp, Intersentia 2001) p. 145.