Greetings

On sheet1 I have a table in A1:B20 that shows 'acceptable pairs', like this:

a mon
a tue
a sun
b mon
b wed
c mon
c thu
c fri
c sat ...etc

On sheet2, users enter similar values in columns C and D. If users enter
'acceptable pairs' as listed in sheet1, that's OK, but if they enter, say
b tue, I don't want excel to accept the pair.

What's the best way to do this?

much thanks for any ideas

Giselle