Frederick,

The Army of the Potomac used two systems of designating flags. The first, in 1862, was created by George McClellan and was down to regimental level with a complicated series of colored flags. The late Howard Madaus wrote about them years ago in Military Collector & Historian, the journal of the Company of Military Historians.

In 1863, Daniel Butterfield created the more well know series of flags from corps to brigade level. These has the symbols of each corps on them with the smaller unit flags being triangular in shape.

If you can find a copy of the Time-Life book "Arms & Equipment of the Union" Madaus from the chapter on Union flags and offers examples for both. West Point Museum has a large collection of these. The McClellan flags are few in number today.

Greg Biggs