Post - captain Post - captain is an obsolete alternative form of the rank of captain in the Royal Navy . The term served to distinguish those who were captains by rank from : Once an officer had been promoted to post - captain , his further promotion was strictly by seniority ; if he could avoid death or disgrace , he would eventually become an admiral ( even if only a yellow admiral ) . In the Royal Navy of the time , an officer might have a rank , but not a command . Until the officer had a command , he was " on the beach " and on half - pay . An officer who was promoted from commander was a captain , but until he was given a command , he was on half - pay . Once the captain was given a command , his name was " posted " in the " London Gazette " . An officer " took post " or was " made post " when he was first commissioned to command a rated vessel — that is , a ship too important to be commanded by a mere commander . Unrated vessels could also in some cases be commanded by post - captains . Being " made post " is portrayed as the most crucial event in an officer ' s career in both Forester ' s Horatio Hornblower series and O ' Brian ' s Aubrey - Maturin series . A junior post - captain would usually command a frigate or a comparable ship , while more senior post - captains would command larger ships . An exception to this rule was that a very junior post - captain could be posted to command an admiral ' s flagship , which was almost always a large ship of the line . The admiral would usually do this to keep his most junior captain under close observation , and subject to his direct supervision . Captains commanding an admiral ' s flagship were called " flag captain s " . One example of an admiral appointing a junior post - captain to command his flagship in this way is the appointment of Alexander Hood to the command of HMS " Barfleur " , flagship of his brother , Admiral Sir Samuel Hood . Sometimes , a high - ranking admiral would have two post - captains on his flagship . The junior of the two would serve as the flag captain and retain responsibility for the day - to - day operation of the vessel . The senior of the two would be the fleet captain , or " captain of the fleet " , and would serve as the admiral ' s chief - of - staff . These two captains would be listed in the ship ' s roll as the " second captain " and " first captain " , respectively . After 1795 , when they were first introduced on Royal Navy uniforms , the number and position of epaulette s distinguished between commanders and post - captains of various seniorities . A commander wore a single epaulette on the left shoulder . A post - captain with less than three years seniority wore a single epaulette on the right shoulder , and a post - captain with three or more years seniority wore an epaulette on each shoulder . ( National Maritime Museum Uniform Collection , see below . ) In the O ' Brian series , Aubrey " wets the swab " — that is , he celebrates his promotion to commander and the acquisition of his " swab " or epaulette with the consumption of copious amounts of alcohol . Note that the term was descriptive only : no - one was ever styled " Post - Captain John Smith " .