Battleships, Battlecruisers
Between the wars, and in World War II, battleships and battlecruisers were given numbers without letters. These numbers were never painted on the sides of the ships.
Monitors
The four monitors changed in May 1940 from the flag superior I to F. The two 'Roberts' class monitors completed after the start of the war were also allocated F.
Cruisers
Heavy cruisers and wartime light cruisers had only numbers:
XXXX	Kent class * Norfolk class York / Exeter Class Leander / Perth Class * Arethusa class Southampton class Dido class Fiji class Minataur class
* The RAN cruisers did carry a flag superior — at the beginning of the war I, changed to D in May 1940.
The older light cruisers started the war with flag superior I, changing to D in May 1940:
	Birmingham class Caledon class Ceres class Capetown / Carlisle class Improved Birmingham / Hawkins class 'D' class 'E' class
Carriers
Most aircraft carriers had numbers without letters.
	HMS COURAGEOUS HMS GLORIOUS HMS FURIOUS HMS EAGLE HMS ARK ROYAL Illustrious class Colossus class (but HMS PIONEER had pennant D.76) Majestic class Hermes class
However, a few carriers started the war with I, which was changed to D in May 1940.
	HMS HERMES HMS ARGUS HMS UNICORN
The following wartime construction aircraft carriers did have pennant letters, namely flag superior D
	Archer class Attacker class Ruler class Audacious class (HMS IRRESISTIBLE — renamed HMS ARK ROYAL while still building in 1945 — did not have a letter.) Gibraltar class
The only four escort carriers built by the British during World War II also had flag D superior.
The seaplane carriers changed in May 1940 from flag superior I to D.
Submarines
Between the wars the pennant number was the reverse of the boats (numerical) name or an identifying number followed by the class letter in the case of a named boat. I.e., numeral pennant and a flag inferior. The flags used were H or L for the 'H' and 'L' classes, P for the 'O' and 'P' classes, R for the 'R' class, F for the 'Thames' class, M for the 'Porpoise' class, S for the 'S' class, T for the 'T' class, and C for the 'U' class.
Then in 1939 all submarines were allocated pennant numbers ending with the N (N flag inferior) — the numbers remained unchanged.
In May 1940, this was changed to flag N superior. Again, the numbers themselves remained unchanged
Early in the war it was decided that submarines built (or acquired) during the war would no longer be given names but only the letter P and a number. (To prevent enemy recognition of new submarines.) Then in 1942, it was once again decided that all submarines would be named.
Destroyers
Historically, pennant numbers were first used in World War I, when large fleet of destroyers were procured by the Navy. The letter depended upon the squadron that the destroyer was in, for example, I, R, N and H were used.
The 'Tribal' class destroyers were originally allocated flag L but this was changed to Flag F in December 1938.
In 1939, old destroyer leaders of the 'Tribal' and 'Shakespeare' classes and the old 'R', 'S', 'V', and 'W' destroyers were allocated D numbers. At the outbreak of World War II destroyers had flags D, H, F superior to their numeral pennants.
Modern destroyers of the 'J' to 'R' and 'Tribal' classes were allocated F, G, H, I and R letters — and flag L used for escort destroyers building ('Hunt' class) or converting ('V' and 'W' classes).
In May 1940 flags D and F were changed to I and G — war construction was also allotted the G until the 'T' class, which adopted the flag letter R.
Early wartime construction was allocated D numbers before names were allocated, and I was assigned to old monitors.
Later, D was allocated to all carriers, and many old light cruisers.
From May 1940, flag superior F was allocated to monitors, gunboats, armed merchant cruisers, boarding vessels and combatant auxiliaries.
Flag superior I was allocated to support, repair and depot ships and also coastal and ocean going antiaircraft ships.
A revision to flag G was made with the 'Weapons' class as well as the cancelled 'G' classes.
As war losses had left many gaps in the original lists, the later 'Battle' and 'D' classes reverted to flag I.

Minelaying vessel
Minelaying vessels were allocated flag superior M. The large fast minelayers received numbers, but only the minor and auxiliary minelayers had numbers painted on. The cruiser minelayer ADVENTURER and the six Abdiel class also had M. Some early 'Flower' class corvettes that were capable of minelaying had flag superior M.
Sloops, Corvettes, and Frigates
At the outbreak of World War II, the following flags superior were worn by the various classes of sloops:
L	Escorts, sloops, patrol vessels (all had previously worn flag P)
N	Minesweepers
J	Surveying vessels (former sloops)
But the corvettes under construction were allocated flag M and the patrol vessels were similarly altered.
But for most of the war, all post 1920's sloops had flag superior U, and K, as in May 1940, with the general reallocation of flag superiors, the following changes occurred:
U	Escorts, sloops (The FLEETWOOD retained flag L)
K	Patrol vessels (All corvettes, small escorts, frigates, and the 'Captain' and 'Colony' class escort destroyers — except PC-74 which was allocated flag Z)
J	All minesweepers and some larger minesweeping trawlers (The HMAS ARARAT received flag K)
Subsequent war construction conformed with the above and the frigates, as they came into service, also received flag K. Exceptions were the former US Coast Guard cutters , which were given flag Y, and the coastal escorts of the 'Kil' class, which received flag 5.
Auxiliary Fighting Vessels
There apparently was no uniform system of categorizing the type or employment of auxiliary merchantile vessels. Although a letter was assigned to each vessel, it was not a definite indication of the its role, as these vessels were subject to constantly changing use.
4	Convoy escorts, auxiliary antiaircraft ships, auxiliary patrol vessels, auxiliary minesweepers, armed yachts
D	Escort carriers, large aircraft transports, transports for crashed aircraft. (Also some special service vessels and auxiliary antiaircraft ships)
F	Small aircraft transports, special service vessels, auxiliary antiaircraft ships
I	Armed merchant cruisers, ocean and armed boarding vessels, auxiliary minelayers. (Changed to flag F or M in May 1940)
FY	smaller trawlers, motor fishing vessels and patrol craft, auxiliary patrol vessels, auxiliary minesweepers, mine destructors, armed yachts
J	Auxiliary minesweepers
M	Auxiliary minelayers
N	Support ships such as paddle minesweepers, surveying vessels, netlayers, AA auxiliaries, boarding vessels and the larger armed yachts (Flag N changed to J in May 1940)
T	Larger trawlers (auxiliary patrol vessels, auxiliary minesweepers, river gunboats)
W	Older trawlers
Z	Cable vessels, ocean going, armed tugs
Auxiliary Support Vessels
The following flags superior were in use at the start of World War II:
I	Depot ship
N	Depot ship
T	Netlayers and net tenders
P	Boom defense vessels
	Repair ships had no flag superior
In May 1940 the system was changed as follows:
F	Depot ships, repair and maintenance ships, base and accommodation ships, (HMS DUNLUCE CASTLE had no flag superior)
J	Depot ships (however, HMS RESOURCE had no flag superior, while HMS BLENHEIM used flag 4)
4	Base and accommodation ships, auxiliary vessels
T	Netlayers and net tenders
Z	Gate, mooring and boom defense vessels
W	Tugs and salvage vessels
X	Miscellaneous vessels
Y	Store carriers
Trawlers, Whalers, and Drifters
The following flags superior were in use during World War II:
4	Auxiliary patrol, balloon barrage, torpedo recovery, wreck dispersal, wreck locating, examination service, armed boarding vessels, and even some antisubmarine and minesweeping vessels.
B	Air-sea rescue
FY	antisubmarine, minesweeping, danlayer, degaussing, mine wiping, mine recovery vessels. Some air sea rescue, auxiliary patrol vessels, balloon barrage, armed boarding vessels.
J	RN: Danlayers; RAN & RCN & RNN: minesweeper trawlers
K	Some antisubmarine trawlers
M	Controlled minelayer trawlers
T	Some antisubmarine and minesweeper trawlers and whalers
U	Some antisubmarine trawlers
Y	Store, fuel, water carriers
Z	Boom defense, boom gate, net layer vessels, naval drifters (the latter wore FY until May 1940)
Coastal Forces
The coastal forces' vessels were not given specific pennant numbers. The boat's (numerical) name itself was worn under the pennant letter.
V	MTB
S	CMB, MA/SB, MGB, SGB, (later changed to flag 8)
Q	ML, HDML
FY	MMS, MFV
J	MAC
P	DCMB, CT