The same countries signed up in 1957 to the Treaties of Rome, which established the European Economic Community, the forerunner of today's European Union.
Then there were nine. After protracted negotiations, Denmark, Ireland and the United Kingdom became members in 1973, taking the EC outside its historic continental heartland - and beginning, some would argue, all the problems of the last 25 years.
Nine became ten in 1981, when Greece, newly liberated from military rule, was admitted, the first in a southwards movement.
Spain and Portugal joined in 1986, taking the total to twelve.
Two became one in 1990, when West Germany unified with East Germany, adding a whole new direction of movement: East.
Fifteen is the current total, after Austria, Finland and Sweden joined in 1995, taking the EU to the borders of Russia and the Arctic Circle.
The next step is to deal with five East European applicants - Poland, the Czech Republic, Hungary, Slovenia and Estonia - and Cyprus.
That would take it to twenty-one members by the beginning of the next century.
But there are six other applicants, which are not deemed ready yet to begin membership negotiations.
Five are from Central and Eastern Europe - Slovakia, Bulgaria, Romania, Latvia and Lithuania. The sixth is Turkey, which is a more distant prospect for membership. That would take membership to twenty-seven. Other past applicants include Norway, which rejected the idea in a referendum, Malta, which has also cooled to the idea, Morocco and Switzerland. If all the applicants were to join, the EU would have 30 members.