The system used to describe electric locomotives uses a series of numbers and letters separated by hyphens to indicate the number of powered and non-powered axles. Non-powered axles, each with two wheels, are represented by the number of them. Powered axles, each with two wheels, are represented by letters as follows: A = 1, B = 2, C = 3 and D = 4. Separate frames are indicated with a + sign at the point of articulation. In this system the GG1 is designated as 2-C+C-2.
On the PRR, steam locomotives were classified using single letters starting with "A" for the 0-4-0 and moving down the alphabet as the number of wheels increased. Ten wheelers (4-6-0) were designated as Class G. The GG1 is an articulated locomotive with two separate frames each having a four wheel leading truck and six drivers or the equivalent of two ten wheelers back-to-back, thus the "GG" and the one indicates that this was the first design of locomotive to have the 2-C+C-2 wheel arrangement on the PRR.