The number of soldiers in a regiment fluctuates, generally depending on casualties and the manpower of the associated army. At its creation, the typical Civil War-era American regiment contained upwards of 1,000 troops. However, at the end of the war, Confederate regiments sometimes had less than 100 troops (barely company-sized).
In the British Army, for most purposes, the Regiment is the largest "permanent" organisational unit. Above regimental level, organisation is changed to meet the tasks at hand. Because of their permanent nature, many regiments have long histories, often going back for centuries; the oldest British regiment still in existence is the Honourable Artillery Company, established in 1537, while the Royal Scots, formed in 1633, is the oldest infantry regiment. (These claims are contested on various points of precedence; see FAQ: Oldest Regiment in the British Army.)
The United States Army was also once organized into regiments, but presently uses the brigade instead, except for cavalry. Although every battalion or squadron is associated with a regiment for historical purposes, the only combat regiments are cavalry regiments which are attached to a corps. These regiments, who are associated generally for historical purposes, can be known as "parent regiments".
Types of regiment in Commonwealth armies
In the British Army and other armies modelled on it, such as Canada's and Australia's, the term "regiment" is used confusingly in two different ways: it can mean a ceremonial grouping or a tactical unit. Ceremonial regiments are not part of the army's day-to-day operational command structure, but regimental ties are maintained by the voluntary efforts of their members. In addition to combat units, other organizations are considered part of the ceremonial regimental family: regimental associations (retirees), bands and associated cadet groups. The parts of a ceremonial regiment have in common such things as a colonel-in-chief (usually a member of the royal family), battle honours (honours earned by one unit of a ceremonial regiment are shared by the whole regiment), ceremonial uniforms, and regimental songs.
Ceremonial armoured regiments are composed of one (usual) or more tactical regiments. For example, the two tactical regiments Le 12e Régiment blindé du Canada and Le 12e Régiment blindé du Canada (Milice) are both part of the ceremonial regiment Le 12e Régiment blindé du Canada.
All the country's artillery is considered part of a single ceremonial regiment. However, there are several tactical artillery regiments. They are designated by numbers, names or both. For example, the tactical regiments 1st Regiment, Royal Canadian Horse Artillery, 10th Field Artillery Regiment, RCA and many others are part of the ceremonial regiment The Royal Regiment of Canadian Artillery.
Ceremonial infantry regiments are composed of one or more battalions. When a regiment has only one battalion, the battalion is usually named exactly the same as the regiment. This means that a battalion's name often contains the word "regiment" despite the fact that it is not a regiment. For example, the battalion The North Saskatchewan Regiment is the only battalion in the ceremonial regiment of the same name. When there is more than one battalion, they are distinguished by numbers, subsidiary titles or both.