I've got a table with 3 pairs of values. Both values in a pair can be null/blank. I want to return rows that have one value in a pair null/blank and one value populated. I've included what I've got but it seems kind of clunky to me. Thanks in advance for any ideas.

Code:
   (ISNULL(PER2a,'') =  '' and ISNULL(PER2b,'') <> '')
OR (ISNULL(PER2a,'') <> '' and ISNULL(PER2b,'') =  '')
OR (ISNULL(PER3a,'') =  '' and ISNULL(PER3b,'') <> '')
OR (ISNULL(PER3a,'') <> '' and ISNULL(PER3b,'') =  '')
OR (ISNULL(PER4a,'') =  '' and ISNULL(PER4b,'') <> '')
OR (ISNULL(PER4a,'') <> '' and ISNULL(PER4b,'') =  '')