AND narrows a search; you
get fewer records, because both concepts
must be present in the records found — here the area in color.
Use AND to combine different concepts
in one search.
OR broadens a search; you get more records
because OR includes each of the concepts separately, as well as
both concepts when found together. OR is often used to link together
related concepts.

NOT narrows a search; you get fewer records
because the second concept
must not be part of the records found. Use NOT to limit your results.

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