I know it well! It is the corner of the French/German FIR at the top right corner of France that juts into Germany, Alsace or Lorraine (can never remember which), the cause of several wars between the participants and regularly swapping hands since 1870, and probably several times before then. The French have it now. Must be time for some fireworks again- if we can kill the EC off it will swap again, and German corner will be something like KRH270/180! German ATC is excellent about letting you graze the corner to save time!