Within military organizations, the use of ranks is almost universal with the Chinese People's Liberation Army of the 1960s and 1970s being a rare example of a military which attempted (quite unsuccessfully) to abolish rank.
The use of formalized ranks came into widespread use with the Roman Legions, after the introduction of reforms by the consul Gaius Marius which were completed around 60 CE. In the new system a legion would be commanded by a legatus, typically a senator given a three-year term. Immediately under the legatus were six tribunes, five senior officers and one nobleman who was headed for the Senate.
The fighting men in the legion were formed into ranks, rows of men who fought as a unit. In the new system these were divided into groups of ten cohorts[?], each consisting of six centuries of 100 men. Each century was led by a centurion, and there were additional centurions who served as scribes and other duties. Centuries were further broken into ten contubernia, of eight soldiers each. Individual soldiers were referred to as miles or legionarius.
In modern terms, the legion represents a battlefield organization about the size of a regiment. As the size and organizational details of the armed forces have grown, additional ranks have been added in order to fill in slots in the organizations. Today the role of the legatus corresponds roughly to General, and a regiment is considered to be one of the smaller organizations within the Army Group he commands, being led by a Colonel. During most of the time since the fall of the Roman Empire the head of the military forces has been the King, often leading in person, but this role, if filled, has since been passed on to dedicated military officers known either as General of the Army, or Field Marshal.
Divisions of the forces has since grown considerably with much larger armies being fielded, which considerably more secondary support roles. A modern army is typically organized in this fashion:
|a||is led by||consists of|
|Army Group[?]||General or Field Marshal||several Armies|
|Corps||Lieutenant General||several Divisions|
|Division||Major General||several Regiments or Brigades|
|Brigade||Brigadier General||several Regiments|
|Lieutenant Colonel||second in command to a Colonel|
|Platoon||First Lieutenant||basic fighting unit|
|Second Lieutenant||First Lieutenant in training|
Many of these ranks are recent additions. The basic unit of an army up to about the 16th century was the Company, which was also known as a Troop in the cavalry and Battery in the artillery. By the 18th century and into the 19th century, the Regiment formed the basic army unit, broken into companies sometimes arranged as Battalions. Brigades and Divisions are more recent additions, with the Brigade replacing the Regiment outright in the British Army.
Commissioned ranks are grouped into three categories:
Non-commissioned officers officially rank beneath commissioned officers, although in many organizations a senior NCO will have formal responsibility and informal respect beyond that of a junior officer.
At the bottom of the ladder is the "private soldier", or private for short.