The Evolution of Forth// ACM SIGPLAN Notices, Volume 28, No. 3. March 1993. ACM SIGPLAN History of Programming Languages Conference,April 1993. Архивиран от оригиналана 9 август 2011.Посетен на 26 април 2016.
Brodie, Leo. Under The Hood//Starting Forth.2nd. Prentice-Hall,1987.ISBN 0-13-843079-9.с. 241.To summarize, there are three kinds of variables: System variables contain values used by the entire Forth system. User variables contain values that are unique for each task, even though the definitions can be used by all tasks in the system. Regular variables can be accessible either system-wide or within a single task only, depending upon whether they are defined within OPERATOR or within a private task.
The Evolution of Forth// ACM SIGPLAN Notices, Volume 28, No. 3. March 1993. ACM SIGPLAN History of Programming Languages Conference,April 1993. Архивиран от оригиналана 9 август 2011.Посетен на 26 април 2016.