Under the Whyte notation for the classification of steam locomotives, 4-2-0 represents the wheel arrangement of four leading wheels on two axles, two powered and coupled driving wheels on one axle, and no trailing wheels. This type of locomotive, often called a Jervis type, was common on American railroads from the 1830s through the 1850s.

Other equivalent classifications are:
UIC classification: 2′A (also known as German classification and Italian classification)
French classification: 210
Turkish classification: 13
Swiss classification: 1/3

The first 4-2-0 built was the Experiment (later named Brother Jonathan) for the Mohawk and Hudson Railroad in 1832. It was built by the West Point Foundry based on a design by John B. Jervis. Having little else to reference, the manufacturers patterned the boiler and valve gears after locomotives built by Robert Stephenson of England.

In England, it had developed from the 2-2-2 design of Stephenson's first Long Boiler locomotive, around 1840, which he had altered to place two pairs of wheels at the front with the outside cylinders between them to improve stability. A few examples of Stephenson locomotives were already in operation in America, so engineers did not have to travel too far to get their initial ideas.