The Royal Garrison Artillery (RGA) was formed in 1899 as a distinct arm of the British Army's Royal Regiment of Artillery serving alongside the other two arms of the Regiment, the Royal Field Artillery (RFA) and the Royal Horse Artillery (RHA). The RGA were the 'technical' branch of the Royal Artillery who were responsible for much of the professionalisation of technical gunnery that was to occur during the First World War. It was originally established to man the guns of the British Empire's forts and fortresses, including coastal artillery batteries, the heavy gun batteries attached to each infantry division and the guns of the siege artillery. The RGA was amalgamated with the RFA in 1924, from which time the only two arms within the Royal Regiment of Artillery have been the Royal Artillery and the Royal Horse Artillery.
