King Ferdinand of Spain organised his army in 1505, dividing it into "colunelas," which translates into "columns" in English. Each colunelas was composed of more than 1,000 men, all lead by the cabo de colunela, or head of the column. These divisions were copied by the French and English armies, who even retained the colunela title for their regimental heads. Over the centuries, the pronunciation of colunela evolved into the current "colonel."
Call a colonel by his rank in social conversation. Whether he is retired or on active duty, proper protocol calls for the use of his title. If you are addressing a letter or invitation to a retired colonel, use only his name and rank, not the branch of service in which he served or his retired status.
Abide by the Department of Defence's directive, which bans the use of a title by retired military personnel when they are involved in commercial enterprises. For example, if a colonel who retired from the U.S. Air Force went to work for Ford Motor Co., she should be addressed as Ms., Miss or Mrs. at work, even though socially she would be addressed as colonel.
Use either the designation Ret. or Retired when addressing official correspondence to a retired officer. Address the envelope using the rank, name followed by a comma, then his branch of service followed by another comma and Ret. or Retired. For example, a letter could be addressed to Col. John Smith, USMC, Retired, or to Col. John Smith, USMC, Ret. Either form of the word is considered acceptable.
Mark all social and official correspondence using the branch of the service from which the colonel retired. For example, if she served in the Marines but retired while a member of the Marine Corps Reserves, correspondence should be addressed to Col. Jane Smith, USMCR, Retired.