"You're thinking of STD, Judy"
When phones were fun! (items on display at the Heusenstamm depot of the Frankfurt Museum for Communications)
Creative Commons Attribution Share AlikeWhen phones were fun! (items on display at the Heusenstamm depot of the Frankfurt Museum for Communications) - Credit: Klaus Nahr

STD stands for subscriber trunk dialling, a telephone system which allows long-distance calls to be made by direct dialling, without operator assistance. This system was introduced into the United Kingdom from 1958 onwards, and involved allocating an individualised STD code to each geographical area within the telephone network; nowadays STD codes are generally known as area codes.

STD also stands for sexually transmitted disease.