AA, the name of a number of small rivers in central and northern Europe, derived from the Celtic ach, or Teutonic aa, flowing water. The most important are: I. A river of the Netherlands, province of North Brabant, which joins the Dommel at Bois-le-Duc. II. A river of Russia, government of Livonia, flowing into the gulf of Riga. HI. A river of France, department of Le Nord, flowing into the North sea near Gravelines.

IV. A river of Switzerland, canton of Aargau, which forms the lakes of Baldegg and Hallwyl, and flows into the Aar.

V. A river of Switzerland, canton of Unterwalden, which flows through the lake of Sarnen, and empties into the lake of Lucerne.

See Aix-la-Chapelle.