[AccessD] Value verses OldValue

I ran into something interesting with the following expression: 

If ctl.Value <> ctl.OldValue Then

It appears that unless you've changed the value, ctl.Value is empty. If
there's been no change, shouldn't Value and OldValue been the same? Why is
Value empty? 

This is in a form's Before Update event, which might matter. 

Susan H.