Eleven countries launched the euro on 1 January 1999
On 1 January 1999 eleven members of the European Union adopted the euro as their official currency. At first it was a currency for accounts and bank transfers only: notes and coins did not arrive until 1 January 2002, so for three years nobody could hold a euro in their hand. Not every EU member took part. One missed the convergence criteria and joined two years later, and three others chose to stay out. The list is fixed: nothing that happens now can add to it or take from it.
Name all eleven first. Then sort them into their three groups.