Younger readers may need an explanation. It used to be that to make a long-distance telephone call, you'd have to use an operator. (And if you're too young even to know what long-distance means, or what an operator is, use Google.) It was an exciting innovation when direct long-distance dialing was introduced. In the UK and Australia, the feature was called "subscriber trunk dialing," abbreviated STD. STD was still a big thing in telephone service when this ad appeared in Australia in 1976. Now you know. Hopefully, I don't have to explain the other meaning of STD.