Pennant number In the modern Royal Navy , and other navies of Europe and the Commonwealth , ships are identified by pennant numbers ( an internationalisation of the term " pendant numbers " which is what they were called prior to 1948 ) . The name pennant number arises from the fact that ships were originally allocated a flag identifying a flotilla or particular type of vessel : for example , in the Royal Navy , the red burgee for torpedo boat s , for torpedo boat destroyer s . By the addition of a number pendant to the identifying flag , each ship could be uniquely identified . A pendant / pennant number thus consists of letters and numbers . Where a letter precedes a number it is known as a " flag superior " and where it is a suffix it is known as a " flag inferior " . Not all pendants / pennants have a flag superior . Royal Navy systems . The system was adopted prior to World War I to distinguish between ships with the same or similar names , to reduce the size and improve the security of communications , and to assist recognition when ships of the same class are together . Traditionally , a pendant number was reported with a full stop " . " between the flag superior or inferior and the number , although this practice has gradually been dropped , and inter - war photos after about 1924 tend not to have the full stop painted on the hull . The system was used throughout the navies of the British Empire so that a ship could be transferred from one navy to another without changing its pendant number . Pendant numbers were originally allocated by individual naval stations and when a ship changed station it would be allocated a new number . The Admiralty took the situation in hand and first compiled a " Naval Pendant List " in 1910 , with ships grouped under the distinguishing flag of their type . In addition , ships of the 2nd and 3rd ( i . e . reserve ) fleets had a second flag superior distinguishing from which naval depot they were manned ; " C " for Chatham , " D " for Devonport , " N " for Nore and " P " for Portsmouth . Destroyers were initially allocated the flag superior " H " , but as this covered only one hundred possible combinations from H00 to H99 the letters " G " and " D " were also allocated . When ships were sunk , their pendant numbers were reissued to new ships . The flag superior for whole ship classes has often been changed while the numbers stayed the same . For example , in 1940 , the Royal Navy swapped the letters " I " and " D " around ( e . g . D18 became I18 and I18 became D18 ) and in 1948 , " K " , " L " and " U " all became " F " , where there was a conflict , a 2 was added to the front of the pendant number . During the 1970s , the service stopped painting pennant numbers on submarines on the grounds that , with the arrival of nuclear boats , they spent too little time on the surface , although submarines do continue to be issued numbers . HMS " Lancaster " was initially allocated the pennant number F232 , until it was realised that in the Royal Navy , form number 232 is the official report for ships that have run aground ; sailors being superstitious , it was quickly changed to F229 . World War II . No flag superior . Pendant number 13 was not allocated . Flag superiors . Pendant numbers 13 were not allocated to flag superiors . The letters J and K were used with three number combinations due to the number of vessels . H T Lenton and J J Colledge ' s book [ British ] " Warships of World War II " ( published in the early 1960s in one or eight parts ) provided even more information on pennant [ sic ] numbers classes and listed each ship ' s pennant [ sic ] number when this was known . Flag inferiors . Flag inferiors were applied to submarines . Royal Navy submarines of the " H " and " L " , and some transferred American vessels , were not issued names , only numbers . In these cases , the pendant number was simply the hull number inverted ( i . e . " L24 " was issued pendant " 24L " ) . Pre - war photos show the pendants painted correctly , with the flag inferior , but wartime photos show that the numbers tend to be painted " backwards " , in that the inferior was painted on as a superior . For obvious reasons , the inferior " U " was not used so as not to confuse friendly ships with German U - boat s . For similar reasons " V " was not used . Pendant numbers 00 — 10 , 13 , and those ending in a zero were not allocated to flag inferiors . Post - 1948 . After World War II , in 1948 , the Royal Navy adopted a rationalised " pennant " number system where the flag superior indicated the basic type of ship as follows . " F " and " A " use two or three digits , " L " and " P " up to four . Again , pennant 13 is not used ( for instance the current " Ocean " - L12 is followed by " Albion " - L14 ; Flotilla bands . 1925 - 1939 . From 1925 , flotilla leader s were issued with but did not paint on pendant numbers . Instead , a broad band deep was painted round their fore - funnel . Divisional leaders wore a pendant number and had a narrower deep band on the fore - funnel , painted from the top . The Mediterranean Fleet wore black leader bands and the Atlantic - later Home Fleet wore white bands . The flotillas wore combinations of bands on their after funnel to identify them . From 1925 the following bands were worn ; 1939 - . When single funnelled destroyers entered the fleet with the J class in 1939 and with an expansion in the number of flotillas , the system was changed accordingly . Single funnelled ships wore a deep band as a flotilla leader . As a divisional leader they had a wide vertical band the same colour as , and extending below , the upper flotilla band . Leaders bands were white for Home Fleet , red for Mediterranean Fleet , and the system of flotilla bands changed to ; Flotilla bands were used throughout the war although war - losses , operational requirements , and new construction broke up the homogeneity of the destroyer flotillas . Vessels were deployed as and when they were needed or available , and were often incorporated into mixed " escort groups " containing a range of vessel types such as sloops , corvette s , frigate s and escort carrier s . A few of the escort groups adopted funnel bands ; others ( like the B7 escort group ) wore letters on their funnels . Post - war . After World War 2 , were no longer identified by bands , but by large cast metal numbers bolted to the funnels . Flotilla leaders continued to display a large band at the top of the funnel , however . Deck codes . Aircraft carriers and vessels operating aircraft have a deck code painted on the flight deck to aid identification by aircraft attempting to land . This is in a position clearly visible on the approach path . The Royal Navy uses a single letter ( typically the first letter of the ship ' s name ) for aircraft carriers and large vessels operating aircraft , and pairs of letters ( usually letters from the ship ' s name ) for smaller vessels . The United States Navy , with its larger fleet , uses the numeric part of the hull classification number ( a system analogous to pennant numbers ) . Deck codes used by contemporary major British naval warships include : International pennant numbers . Several European NATO and Commonwealth navies agreed to introduce a pennant number system based on that of the Royal Navy . The system guarantees that , amongst those navies and other navies that later joined , all pennant numbers are unique . The United States , as mentioned before , does not participate in the NATO system ; its ships are identified by a unique hull classification symbol . Participating countries , with their assigned number ranges , include : The NATO pennant number system added the " Y " ( for " yard " ) symbol for tugboats , floating cranes , docks and the like .