Originally Posted By: TheFallibleFiend

With alphanumeric codes on the computer, it's often difficult to distinguish the numeral '0' from the letter 'O'. On the old teletypes, they used the null symbol, Ø, an 'oh' with a diagonal slash through it to distinguish it from the letter. You can see the null symbol over the letter 'P' here: http://www.flickr.com/photos/ajmexico/2766045525/#/photos/ajmexico/2766045525/lightbox/

(Teletypes only used capital letters, btw, which is why you don't see a "caps lock" key.)


The teletypes we had in the '70s used Ø and 0 for the letter and the number, but there was no consistency from one teletype to another nor was there any necessary consistency between the keyboard and the type cylinder on any given machine. I was a student lab assistant in the computer room (time-sharing on a Xerox Sigma 6 mainframe) and became something of an expert in distinguishing between the number and the letter in students' printouts.