Regimental Corporal Majors are perculiar to the Household Cavalry, which forms part of the Guards Division. It dates back to Victorian times and is all to do with Queen Victoria not wanting to confuse Sergeants with Servants. That is why Senior Ranks are Corporal of Horse, Staff Corporal of Horse, Squadron Corporal Major and Regimental Corporal Major. If they are in a Garrison appointment they will then be the Garrison Corporal Major as opposed to the Garrison Sergeant Major. Confused- I was for 22 years!
