On Tue, 3 Apr 2012 12:09:38 +0100, Roland Perry
wrote:

In message , at 07:45:42 on
Tue, 3 Apr 2012, Charles Ellson remarked:
Fixed telephones outwith the "director
areas" (those like Greater London, Glasgow etc. which used the first
three letters of the exchange as the code and where the exchanges used
translation) did not have letters except by accident

I agree that (Inner) London Exchanges had three-letter abbreviations,
but very many provincial exchanges had two-letter abbreviations, plus an
index digit, as a mnemonic...

So Cheltenham was CH2 ( 0-24-2 )
Chichester was CH3 ( 0-24-3 )
Chester was CH4 ( 0-24-4 )
Chelmsford was CH5 ( 0-24-5 ) etc

Not "director areas" though. The style of the STD codes was fairly
deliberate (rather than e.g. a helpful aid for use within the GPO) but
the future use of letters was dropped before STD working left the
trial stage.