GeoDuo

GeoDuo shows one line, the shape of a land border taken from Natural Earth map data, with nothing else on the map to give it away. You can rotate and zoom the line, then pick two countries from the lists and press Check. The order does not matter.

You have six attempts. If one of your two countries is right it shows in green and can stay in your next answer. Each wrong answer unlocks a hint, in this order: the regions, the length of the border, the outline of one country, the outline of the other, and finally both initials. There is no distance scale on the map, so the length hint stays useful.

Each challenge is numbered and dated, changes at 00:00 UTC and is the same for every player; the game runs through all 342 borders in its data before any repeats. Maritime borders are left out. Your progress, streak, statistics and an atlas of the borders you have solved are kept in the browser, earlier days can be played from the history, and you can share your guesses without revealing the countries. The site is in English, Spanish, French, Portuguese, German, Italian and Catalan.