A Captain is a chieftain or head of a unit. The title comes from the Latin word capitaneus that meant chieftain, which in turn came from an older Latin word caput that meant head. It would seem that a Captain could head a unit of any size but as armies evolved his post came to be at the head of a company, which by the Sixteenth Century was usually 100 to 200 men. That seemed to be the number one man could manage in battle. There appear to have been Captains leading Italian soldiers in the Tenth Century. In the Eleventh or Twelfth Century, British warships carried groups of soldiers commanded by Captains to do the fighting. The Navy's rank of Captain came from that practice, which I will describe later in the section on the Navy Captain.
Captains were company commanders in the British, French and other armies for centuries. They carried on that job in our Army and Marine Corps from 1775 to the present. In the Air Force, some Captains command some squadrons, which are about the equivalent of companies.
Army Captains got their rank insignia of two bars in about 1832 at the same time the First Lieutenants got one bar. The bars were gold except for the Infantry officers who wore silver bars until 1851. The two bars originated a few years earlier when Captains and Lieutenants both wore plain epaulettes whose differences were mostly in the size of the fringes. To help distinguish between the two ranks, Captains wore two strips or "holders" of gold or silver lace across the epaulette straps while Lieutenants wore one strip. In 1872 Captains changed to silver bars. These were two separate bar embroidered onto shoulder straps or epaulettes. The "railroad tracks" used by Captains today appeared when officers started using metal pin-on rank insignia on their khaki or olive drab uniforms during or shortly after the Spanish-American War.