The enlargement of the eurozone is an ongoing process within the European Union (EU).
 All member states of the European Union, except Denmark and the United Kingdom which negotiated opt-outs from the provisions, are obliged to adopt the euro as their sole currency once they meet the criteria, which include: complying with the debt and deficit criteria outlined by the Stability and Growth Pact, keeping inflation and long-term governmental interest rates below certain reference values, stabilizing their currency's exchange rate versus the euro by participating in the European Exchange Rate Mechanism (ERM II), and ensuring that their national laws comply with the ECB statute, ESCB statute and of the Treaty on the Functioning of the European Union.
 The obligation for EU member states to adopt the euro was first outlined by the Maastricht Treaty of 1992, which became binding on all new member states by the terms of their treaties of accession.
The signing of the Treaty of Lisbon took place in Lisbon, Portugal, on 13 December 2007.
 The Government of Portugal, by virtue of holding Presidency of the Council of the European Union at the time, arranged a ceremony inside the 15th century Jerónimos Monastery, the same place Portugal's treaty of accession to the European Union (EU) had been signed in 1985.
 Representatives from the 27 EU member states were present, and signed the Treaty as plenipotentiaries, marking the end of negotiations that began in 2001.
 In addition, for the first time an EU treaty was also signed by the presidents of the three main EU institutions.
 After the main ceremony, the heads of state and government took a ride on a decorated Lisbon tram together, symbolising the brotherhood of European countries on the path of European integration.
Operation Triton is a border security operation conducted by Frontex, the European Union's border security agency.
 The operation, under Italian control, began on 1 November 2014 and involves voluntary contributions from 15 other European nations (both EU member states and non-members).
 Current voluntary contributors to Operation Triton are Croatia, Iceland, Finland, Norway, Sweden, Germany, the Netherlands, France, Spain, Ireland, Portugal, Austria, Switzerland, Romania, Poland, Lithuania and Malta.
 The operation was undertaken after Italy ended Operation Mare Nostrum, which had become too costly for a single country to fund; it was costing the Italian government €9 million per month for an operation that lasted 12 months.
 The Italian government had requested additional funds from the other EU member states but they did not offer the requested support.
The European Central Bank (ECB; German: "Europäische Zentralbank (EZB)", French: "Banque centrale européenne (BCE)") is the central bank for the euro and administers monetary policy of the eurozone, which consists of 19 EU member states and is one of the largest currency areas in the world.
 It is one of the world's most important central banks and is one of the seven institutions of the European Union (EU) listed in the Treaty on European Union (TEU).
 The capital stock of the bank is owned by the central banks of all 28 EU member states.
 The Treaty of Amsterdam established the bank in 1998, and it is headquartered in Frankfurt, Germany.
 s of 2015 the President of the ECB is Mario Draghi, former governor of the Bank of Italy, former member of the World Bank, and former managing director of the Goldman Sachs international division (2002–2005).
 The bank primarily occupied the Eurotower prior to, and during, the construction of the new headquarters.
The European Union (EU) is a political and economic union of 28 member states that are located primarily in Europe.
 It has an area of 4,475,757 km2 , and an estimated population of over 510 million.
 The EU has developed an internal single market through a standardised system of laws that apply in all member states.
 EU policies aim to ensure the free movement of people, goods, services, and capital within the internal market, enact legislation in justice and home affairs, and maintain common policies on trade, agriculture, fisheries, and regional development.
 Within the Schengen Area, passport controls have been abolished.
 A monetary union was established in 1999 and came into full force in 2002, and is composed of 19 EU member states which use the euro currency.
Development cooperation between the European Union (EU) and the countries of the African, Caribbean and Pacific Group of States (ACP) celebrated its 50th anniversary in 2007.
 Although bilateral relations have always been and still remain one of the main features of modern development cooperation, it was the Treaty of Rome in 1957 which first established a collective European development policy.
 The Treaty of Rome granted associated status to 31 overseas collectivities and territories (OCTs) and provided for the creation of a European Development Fund (EDF) intended to grant technical and financial assistance to the countries which were still under European rule at the time.
 More significantly, however, by means of the Treaty of Rome the six member states of the European Economic Community were expressing solidarity with the colonies and OCTs and committed themselves to contribute to their prosperity.
 The EDF has to date been funded outside the EU budget by the EU Member States on the basis of financial payments related to specific contribution shares, or “keys”, which are subject to negotiation.
 The EDF is currently the only EU policy instrument that is financed through a specific key that is different from the EU budget key, and which reflects the comparative interests of individual Member States.
The European Union value added tax (or EU VAT) is a value added tax on goods and services within the European Union (EU).
 The EU's institutions do not collect the tax, but EU member states are each required to adopt a value added tax that complies with the EU VAT code.
 Different rates of VAT apply in different EU member states, ranging from 17% in Luxembourg to 27% in Hungary.
 The total VAT collected by member states is used as part of the calculation to determine what each state contributes to the "EU's budget".
PIGS or PIIGS or PIIGGS is an acronym used in economics and finance.
 The PIGS acronym originally refers, often derogatorily, to the economies of the Southern European countries of Portugal, Italy, Greece, and Spain.
 During the European debt crisis, the term was also increasingly used to refer to the economies of Portugal, Ireland, Greece, and Spain, four EU member states that were unable to refinance their government debt or to bail out over-indebted banks on their own during the crisis.
European Dactyloscopy (EuroDac) is the European Union (EU) fingerprint database for identifying asylum seekers and irregular border-crossers.
 Asylum applicants and irregular border-crossers over the age of 14 have their fingerprints taken as a matter of European Community law.
 These are then sent in digitally to a central unit at the European Commission, and automatically checked against other prints on the database.
 This enables authorities to determine whether asylum seekers have already applied for asylum in another EU member state or have illegally transited through another EU member state ("principle of first contact").
 The Automated Fingerprint Identification System is the first of its kind on the European Union level and has been operating since 15 January 2003.
 All EU member states currently participate in the scheme, plus three additional European countries: Norway, Iceland and Switzerland.
The European debt crisis (often also referred to as the Eurozone crisis or the European sovereign debt crisis) is a multi-year debt crisis that has been taking place in the European Union since the end of 2009.
 Several eurozone member states (Greece, Portugal, Ireland, Spain and Cyprus) were unable to repay or refinance their government debt or to bail out over-indebted banks under their national supervision without the assistance of third parties like other Eurozone countries, the European Central Bank (ECB), or the International Monetary Fund (IMF).

Question: During Which debt crises did PIGS  used to refer to the economies of Portugal, Ireland, Greece, and Spain, four EU member states
Answer: The European debt crisis