Pennant Numbers, NATO

NATO adopted a pennant number system that was based on that of the Royal Navy but which streamlined the numbering schemes and assigned specific number ranges to participating countries. This ensured that ships from the same class would have consecutive numbering (with some exceptions explained below), and that the same pennant number would not be assigned to different ships from different countries. Notably, NATO's two non-European members, the US and Canada, did not adopt the system; the US retained its unique hull classification symbols while Canada retained the Royal Navy system until switching to a US-style hull classification in the 1960s (as did another close US ally, Australia).

NATO's flag superiors are almost identical to those of the Royal Navy's 1948 scheme. They also usually override the national designation in order to be used for comparision purposes. So, for example, the French no longer use the term destroyer (contre-torpilleur); all ships of destroyer and frigate size are known as frigates (frégate). However, the French Navy gives all destroyer-sized ships the flag D rather than F. There are a few exceptions to this. The Franco-Italian FREMM class of ships (known as the European Multi-Mission Frigate) are designated as destroyers (D) by the French rather than frigate (F) even though the Italian ships are actually slightly larger! Aside from these exceptions, the flags are often useful to differentiate between ships of similar size, such as corvettes (F) and offshore patrol vessels (P).

NATO's pennant numbering system assigns a number range to participating navies in batches of one hundred. Within these ranges, each navy usually assigns a consecutive numbering system to the ships in that class. Most of the time, the first ship in the class is assigned a number that is a multiple of ten. Given that nowadays most European ship classes are built in small numbers compared to the past, this means that classes can be differentiated by their second digit. For example, French destroyers are assigned the 600 batch of the D flag superior which means any such ship can received a pennant number between D600-D699. The George Leygues-class of guided missile destroyers began with D640 for the lead ship, continuing sequentially until the last ship of the class, D646. From here the numbering jumps to D650, which is the first ship of the Aquitaine-class (FREMM) which is expected to be eight-strong, therefore the last ship is D657. But class numbers are not always consecutive: the two-strong Horizon-class was built between the Leygues and Aquitaine ships and are assigned D620-D621. Note that one country can have various batches (Britain has the 00 and 100 batches for destroyers), and that the same batch can be assigned to more than one country (Denmark and Norway share the 300 batch for frigates).

Not all classes of ships begin with a different second digit. For example, the Le Redoubatle-class of ballistic nuclear submarines are assigned S610-S615. These were followed by the Triomphant-class which are assigned S616-619. Not all classes begin with a zero either: the Rubis-class of nuclear attack submarines begin with S601 and there is no submarine in the French navy that has been assigned S600. Some exampls of non-consecutive numbering still exist, such as Britain's Duke-class (Type 23) frigates which began with F230-F239 but then jumped to F79-83 in a later batch of ships. Lastly, tradition and superstition remain present in numbering. The Type 23 ships skipped F232 for HMS Lancaster because F232 is the name of the Royal Navy form that needs to be filled out whenever a ship is grounded (this oversight was noted just shortly before the ship was commissioned; pictures of the ship being built with F232 on the hull exist). As a result, HMS Lancaster was re-assigned F229. The "unlucky" number 13 is also often skipped, as was the case when the amphibious ship HMS Albion was assigned L14 while its immediate predecessor, HMS Ocean, was L12. As can be seen, despite efforts at normalizing pennant numbers, there is still a large degree of arbitary assignments although on the whole it is a huge step up from the Royal Navy's considerably chaotic old schemes.

Finally, it should be noted that all pennant numbers are at least two digits long. Hence Britain's first post-NATO carrier, HMS Eagle, was assigned R05. Pennant numbers can also be recycled once a ship has been decommissioned; the future British carrier HMS Prince of Wales (currently under construction) will have pennant number R09, the same as the Cold War-era HMS Ark Royal. In no case, however, can an active NATO ship have the same pennant number as another.