Hypothetical exchanges exist where part of an exchange is given a separate identity for operational purposes.
Sometimes this is done to provide special facilities for large exhibitions that represent a 'community' of their own. Examples include Air Show (the numbers of which are in fact Farnborough numbers) and Royal Show (these are in fact Coventry numbers), both being notional exchanges for use during exhibitions. In the same way, DREadnought was used only for lines in the Earls Court and Olympia exhibitions in London (it was equivalent to dialling FREmantle, a 'real' exchange).
Another use of hypothetical names came into play when subscribers disliked the name of their exchange, as in the case of LEYtonstone and KEYstone. Residents of Upper Leytonstone who were originally on Walthamstow (manual) exchange objected to being given numbers on LEYtonstone, a place they considered distinctly unfashionable. Large numbers of exchange lines were needed during exhibitions at Earls Court and many of these were provided from FULham exchange. Since FULham is not one of London's most fashionable districts, for these lines the name DUKe was created as an alias of FULham. In east London there was so much opposition to the proposed name of BEThnal Green that the alternative name of ADVance was created. In all three cases, the letters of both names of the pairs correspond to the same numeric code digits.
The last use of hypothetical exchanges is to cater for future expansion. A district may show signs that its telephone 'population' is due to grow before long but is not yet ready to support an exchange of its own. For customers in this area of expansion a separate group of numbers is created on the parent exchange, with a separate name and access code being created. For the time being it is arranged that anyone dialling the new exchange code is re-routed to the old exchange. When, however, a separate exchange is warranted for the new district the customers on the hypothetical unit are transferred to identical numbers on the new exchange and the access arrangements changed over. This clever system avoids disruption at the time of changeover, and the hypothetical subscribers are never the wiser!
Whilst on the subject of hypothetical exchanges, it is usual (at least in London) for a director exchange to commence life as a hypothetical unit on its parent or predecessor. E.g. FITzroy was actually part of level 0 of MOUntview before it took off on its own to cover the same area. (They nearly always seem to be levels 9 or 0, for the obvious reasons that the top levels may well have not been used up by the time that the next unit is under way).
COLmore in Birmingham was similar, only here the real COLmore was never built and COLmore numbers later reverted to their true identity on CENtral.
You do get some strange anomalies, however. For instance 01- 935 (WELbeck) has as its second unit 486 (HUNter). This in turn spawned a third unit, 487, which is (or at least was) hypothetically worked on 289 (LORds) - 286 CUNningham's second unit, which is quite a few miles away. I suppose that these days, with the ability to feed umpteen hundred channels up and down a co-ax or fibre, it really doesn't matter!
Gatwick has always been hypothetical on Crawley (not Horley). The airlines always wanted Gatwick to be in the London charge zone but the Post Office resisted: it would have made a huge bulge in the map south of Redhill (which is a local call from London). In the event, Gatwick Airport (or Gatport Airwick if you insist!) paid for a lot of out-of-area lines from London, which for some strange reason were on AVEnue exchange.
From an article originally published in Telecom Heritage, the journal of the Telecomms Heritage Group.
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