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