change makes about 200,000 new numbers available for each area code in which the change has been made. Some telephone service providers differentiate ordinary calling costs using the relevant area code(s). Until STD was introduced, only telephone operators could connect calls over the trunks (long-distance links between major exchanges). A subscriber would have to dial 0 for the operator and then request a long-distance call. As STD was introduced area by area the meaning of the 0 changed; it was now the trunk prefix used to raise a call automatically to the trunk dialling level, what telephone companies now call a National call. The new code of 100 was introduced for calling an operator. The leading 0 is not part of an area code, which is why international callers dialling into the UK must not dial it. For historical and operational reasons, the area code plus local number can have varying total and composite digit lengths, but as a rule they do not exceed 11 digits in combined length. For readability, and to distinguish geographic location, telephone numbers are often spoken, displayed and published with a gap between the area code and local number, or with the area code in (parentheses). Problems may occur for the reader when this spacing or formatting is incorrectly applied by the publisher. London numbers History of the confusion When the UK's original STD codes were allocated in the late 1950s, London was given the code 01. Relatively few subscribers could dial trunk calls, so the 01 code was not generally included as part of the published telephone number. In the early 1960s, London telephones still had exchange names so the first three letters had to be dialled before the four-digit local number, e.g. ABBey 1234 (London Transport). In the mid-1960s, All Figure Numbers (AFNs) were introduced in London and five other large cities because the number of meaningful letter combinations was becoming exhausted (and the letters mapped to the numbers in a slightly different way to the US, which would be problematic for international dialling). The STD codes were incorporated as part of the AFN, ABBey 1234 becoming 01-222 1234. Callers in London were still instructed to dial only the seven digits (those after the hyphen) when calling other London numbers because a charging error would result if the 01 code were dialled, with the call being charged as a national call rather than