The tally on a sailor's cap is a ribbon usually bearing the name of a ship or some other establishment to which he belongs.
Practice varies with each navy, though a conventional tally is black, with a gold or yellow inscription. The inscription may be simply a ship's name (e.g. "H.M.A.S. ARRERNTE"), the name of the navy ("MARINE NATIONALE") or a longer name such as "Red Banner Baltic Fleet" ("КРАСНОЗНАМЕН. БАЛТ. ФЛОТ"). During World War II the ship's name would often be omitted from the tally (leaving just "H.M.A.S", for example) as a precautionary measure against espionage.
Likewise the manner a tally is fastened onto the cap varies with each navy. For example, the British tie it into a bow on the left side; the Germans and Russians tie it at the back, leaving behind a pair of streamers; while the French stitch it onto the cap like an ordinary cap band.