The Marine Corps has the "RULE of THREE." I believe it was invented during the Anna Wars by Edison who invented the fire team concept that is used today. In a nutshell, the rule is this: each Marine has three things to worry about. Three men to a fire team commanded by a Corporal (so there are actually a total of four on the team, when you count the team leader). Three fire teams to a rifle squad commanded by a sergeant. Three rifle squads to a platoon commanded by a Lt.. Three rifle platoons to a company commanded by a Capt. Three companies to a battalion commanded by a Lt Col. etc.
Team: Four individual Marines assigned to a specific team (Three team members, plus the team leader).
Squad: Three Teams are assigned to a specific squad.
Platoon: Three squads are usually assigned to a specific platoon.
Company (or Battery): Three platoons are assigned to a Company (sometimes called a battery). The Company/battery is the lowest level of command with a headquarters element (example, a Company Commander, or Company First Sergeant).
Battalion: Three companies/batteries are assigned to form a battery a battalion.
Regiment: Three battalions form a Regiment (Sometimes called a Brigade).
Division: Three Brigades are assigned to make up a Division.
Marine Corps: Three or more divisions make up the Marine Corps.
MEU: In addition to the above, there are also MEUs (Marine Expeditionary Unit). With a strength of about 2,200 personnel, the MEU is normally built around a reinforced battalion, a composite aircraft squadron, and by a MEU Service Support group. Commanded by a colonel, the MEU is employed to fulfill routine forward deployments with fleets in the Mediterranean, the Western Pacific, and...