Brain Game: The European Seven

Feel free to consult a European map to assist you with today's Brain Game; a quick examination may help lead you to the answer. Good luck!

Geographically, what do these seven independent European nations - but no others on the continent - have in common? Denmark, Holy See (Vatican City), Republic of Ireland, Monaco, Portugal, San Marino, United Kingdom.

Here is the SOLUTION.

THE SOLUTION:

They are the only independent nations in Europe that border only ONE other independent European nation:

Denmark borders only Germany;
Holy See borders only Italy;
Republic of Ireland borders only the United Kingdom;
Monaco borders only France;
Portugal borders only Spain;
San Marino borders only Italy;
United Kingdom borders only the Republic of Ireland.