This system was introduced by the VDEV, a group of German railways,
at the beginning of the 20th century. At least for Diesel
and electric locomotives, it is now used internationally.
A B ..
Capital letters stand for a group of driven axles in the
same frame. A = 1 axle, B = 2 axles, and so on.
o
A small 0 or o after a letter means that every axle in
this group is driven by an individual motor.
1 2 ..
Arabic numbers stand for a group of non-driven axles in
the same frame. 1 = 1 axle, 2 = 2 axles, and so on.
( )
Brackets are used if driven and non-driven axles are in
the same bogie.
'
An apostrophe after a letter, number or bracket stands
for a bogie or non-rigid axle.