The A Device is a decoration of the United States military which is presented as an attachment to the American Defense Service Medal and the Air Force Overseas Service Ribbon. The A Device is a bronze colored letter "A", pinned to the center of both awards.
The American Defense Service Medal was the first decoration to use the A Device and the award was originally known as the "Atlantic Device" and also as the "Axis Device". The A Device was awarded to any member of the United States Navy who, while deployed to the Atlantic Ocean between June 22 and December 7, 1941, engaged in armed conflict, or potential armed conflict, with naval forces of the German Kriegsmarine. Those having done so were awarded the American Defense Service Medal with A Device, the intent being to recognize those who had participated in the "undeclared war" when the United States of America was assisting Britain with war convoys and German U-Boat interdiction.
The A Device became obsolete after the Second World War and did not appear again until the year 2002. At that time, the United States Air Force declared that the A Device would be awarded to those who had received the Air Force Overseas Short Tour Service Ribbon for tours of duty in arctic locations such as Northern Greenland and Alaska*.
Now known as the "Arctic Device", the A Device is awarded as an attachment to the Air Force Overseas Short Tour Service Ribbon. The A Device is not authorized for any other United States military decorations.
Note, however, that the two representations of the A Device are different. The Atlantic Device does not have "feet" (sans-serif), but the Arctic Device does have serifs.
*Although Alaska has locations within the Arctic Circle, there are currently no bases within that region. Only Thule Air Base qualifies for this device, as of 8 February 2007.