Seventeen Cavalry Reserve Regiments were formed by the British Army on the outbreak of the Great War in August 1914. These were affiliated with one or more active cavalry regiments, their purpose being to train replacement drafts for the active regiments. In 1915, the 3rd Line regiments of the Yeomanry were also affiliated with the Cavalry Reserve and, in September 1916, the Household Cavalry Reserve Regiment was formed in Windsor, supplying replacements to the dismounted Household Battalion. In 1917, the regiments underwent major reorganization, being reduced to ten in number.