It was based on an reverse Polish notation (RPN) form of the language Lisp, combined with some ideas from Combined Programming Language (CPL).
COWSEL was initially implemented on a Ferranti Pegasus computer at the University of Leeds and on a Stantec Zebra at the Bradford Institute of Technology.
Later, Rod Burstall implemented it on an Elliot 4120 at the University of Edinburgh.
Reserved words (keywords) were also underlined in the original printouts.
Popplestone performed syntax highlighting by using underscoring on a Friden Flexowriter.