Four Inch Course

The name of the course derives from the regulations for the 1908 Tourist Trophy adopted by the Royal Automobile Club, which limited the competitors' engines to a cylinder-diameter of four inches.

The Four-Inch Course was adopted by the Auto-Cycle Club for the 1911 Isle of Man TT Races.

The Four Inch Course was based on a number of public roads closed for the duration of racing, including: The Four Inch Course was adopted by the Auto-Cycle Club for the 1911 Isle of Man TT Races and the start-line was moved to a level section of the Quarterbridge Road between Selborne Drive and the 1st Milestone/Alexander Drive, becoming known as the Snaefell Mountain Course when used for motorcycle racing.

[2] From the 1920 Isle of Man TT races, changes were made to the Snaefell Mountain Course and competitors turned left at Cronk-ny-Mona following the primary A18 Bemahague Road to Governor's Bridge, then to a new start/finish line on Glencrutchery Road which lengthened the course from the pre-World War I length of 37.50 miles (60.35 km) to 37.75 miles (60.75 km).

More changes to the course followed for the 1923 Isle of Man TT races with the adoption of two small sections of private road between Parliament Square and May Hill in Ramsey.