Perhaps you should try some spelling variations:
"principal states" » "principal statutes", "principal stress", "principal scenes"
"principal series" » "principal cities", "principal secretaries"
"principal seats" » "principal acts"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("principal states") OR ("principal seats"))) OR ("principal series")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"principal states\") OR (\"principal seats\"))) or (\"principal series\"))".
• Removing quotes may allow a broader search: (((( principal states ) OR ( principal seats ))) or ( principal series )).
• Adding a wildcard symbol may retrieve word variants: (((("principal states") OR ("principal seats"))) or ("principal series"))*.