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[[File:GWR_Diagram_AA15.jpg|thumb|300px|right|GWR Wagon Diagram AA15, 20 Ton 'Toad' brake van, August 1918]]
'''GWR Diagram codes''' were introduced by George Jackson Churchward to allow easy reference to different types of coaching stock. The Diagram Code for each type consisted of one or two letters which represented the general category, and a number which denoted particular design of that type, such as A28 or DD3.<ref>[https://en.wikipedia.org/wiki/Coaches_of_the_Great_Western_Railway#Diagram_codes Wikipedia]</ref>. The Diagram Index for wagons was first prepared circa 1905, followed by a similar index for wagons carriages around 1910.<ref>[[Bibliography#Other References|Atkins, Beard & Tourret (2013)]] pp. 490-494.</ref>
The numbering schemes used for coaching stock and wagons ran in parallel, so the same codes appeared in both. Thus on the SVR, [[GWR 1257 Siphon G Bogie Milk Van | Siphon G 1257]] was built to Coaching Stock diagram O11 while [[GWR 13154 5 plank Open Goods Wagon | 5-plank Open Wagon 13154]] was built to Wagon diagram O11. The GWR Diagram Book which listed the diagrams typically allocated a page for each diagram which showed its major dimensions and characteristics.