The first compound-compression locomotive with an articulated pair of drive wheel assemblies was designed by Swiss engineer Anatole Mallet (pronounced "Malley") in France. The front driver assembly included two low-pressure cylinders. The rear driver assembly included two high-pressure cylinders. The single boiler was rigidly attached to the rear driver assembly.

Mallet locomotives in the USA followed the design created by Anatole Mallet and were called Mallet locomotives as a result. Like Anatole's original design, these locomotives used compound expansion where steam was first used the the two high-pressures cylinders and then exhausted to be used a second time in the two larger low-pressure cylinders in the front of the locomotive.

The USA later experimented with the same basic design but with four high-pressure cylinders. These were still articulated locomotives but were no longer true "Mallets" because they used simple expansion instead of compound expansion. Unfortunately, no good name for this design ever emerged, and they tended to be loosely called 'Mallets' as well.

Some people loosely use the term "Mallet" to describe any articulated locomotive. However, I prefer to be more specific and use the term "articulated" to describe a single-expansion, articulated steam locomotive. Periodically, I hear a Big Boy or Yellowstone referred to as a "Mallet" type locomotive. Technically, this is not true. Anatole Mallet designed a steam locomotive with two important characteristings. This locomotive

1. was articulated, and
2. used compound expansion (high and low pressure cylinders)

The Big Boys, Yellowstones, as well as many other articulated steam locomotives, used simple expansion (high pressure cylinders all around), and thus, according to the strictest definition of the term, are not true mallets. Look at this picture of a Big Boy. Notice that the front and rear cylinders are the same size. Because they are both the same size, they operate using the same (high) pressure.

Articulated locomotives such at the N&W Y class did use compound expansion and therefore are true "mallets". Look at this picture of a 2-8-8-2. Notice the much larger (low pressure) front cylinders. That is your best clue that the locomotive is a true mallet.