The PG class was introduced in December 1899 by Charles Clifford with 4feet 7inches (1.40m) driving wheels and 18+1⁄2 by 24 inches (47cm ×61cm) cylinders.[2] The QG's introduced in 1904 were 1foot 6inches (0.46m) longer than the PG with 18+1⁄2 by 26 inches (47cm ×66cm) cylinders weighed in 2 tons heavier.[3] The key changed with the QLG class introduced in 1906 was a 4feet 9inches (1.45m) wide boiler.[4]