Thanks very much for the reply. - However ...
The people who will use the spreadsheet are older people who are not comfortable with computers. They are used to using a paper form where they enter a dash if there is no data ( to show that it hasnt been lost or forgotten but really isnt there ). So I've made this spreadsheet absolutely identical to the paper one so that they need zero training to use it and to make the transition as smooth as possible for them. That's why I'd ideally like to trap the keystrokes and change the behaviour of the minus sign in that way. I understand if it's a limitation of excel though. I might have to write it as a web page with javascript,php and mysql if excel can't do it. What do you think ?