The Janney Coupler is a semi-automatic railway coupler. The earliest commercially successful version of the knuckle coupler, it was patented by Eli H. Janney in 1873 (U.S. Patent 138,405).

In the United Kingdom, where it is fitted to some rolling stock, mostly for passenger trains, it is also known as a Buckeye Coupler, possibly originating from the coupler's manufacture as early as 1890 by the Buckeye Steel Castings firm in Columbus, Ohio, US.

The AAR/APTA Type E, Type F, and Type H tightlock couplers are all compatible knuckle couplers, but are employed on specific rail car types (general freight, tank cars, rotary hoppers, passenger, etc.).

Prior to the formation of the AAR (Association of American Railroads) these were known as MCB Couplers (Master Car Builders Association). In 1934 the MCB reconstituted itself into the AAR.

Early knuckle couplers using a variety of proprietary head designs, but all using the most up-to-date MCB contour when cast, were the MCB, Janney (later version), Tower, Sharon, Climax, Gould, Burns, Miller, and as many as 100 others.

In 1913 the American Steel Foundries (ASF) had developed the new Type "D" coupler that was accepted as the standard coupler for the United States, and no new equipment could be built using any other coupler. This standard design ended the market for couplers with a proprietary head design, which were common in the MCB days, to all but those exported from the US to other countries not governed by the AAR standards.

The Alliance coupler, named after the ASF-owned foundry in Alliance, Ohio, was developed as a lighter build than the AAR Type "D" coupler, and was marketed by the Amsted Corporation, the parent corporation of ASF, as the "Standard Coupler for the World". It was and is still the most used knuckle coupler in the world. The modern Alliance coupler still uses the modern AAR-10 and/or -10A contour, as well as others, but cannot be used in the US for "interchange" rolling stock

Brand names of the now standard AAR Type "E", "F" and "Tightlock" couplers are ASF, Buckeye (now known as Columbus Castings), and McConway & Torley.

The Interlocking contour of knuckle couplers was the first aspect to be standardized. In the MCB years, prior to about 1910, and as early as the first Janney in 1873, there were many proprietary "head" designs, and many MCB standard contours, which were constantly evolving and changing (as the approved standard contour for new couplers) every few years. In about 1910 the producers were all using the then standardized MCB-10 contour, soon to become the AAR-10. In the 1930s the AAR Type "D" was improved and became the Type "E"; the contour, however, stayed the same. A few years later the 10 contour was modified into a then optional standard called the 10A contour.

The most modern contour, for a plain Type "E" knuckle coupler, is still the AAR-10 and -10A, which are indistinguishable from the 1910 era MCB-10 contour. The same MCB-10 contour has been an approved standard for interchange service for over 100 years, with only the slightest dimensional changes.