Boolean Operators

Using AND, OR, and NOT to Refine your Keyword Search

Boolean searching is based on an algebraic system of logic formulated by George Boole, a 19th century English mathematician. In a Boolean search, keywords are combined by the operators AND, OR and NOT to narrow or broaden the search (you do not have to enter them in capitals).

The operator AND narrows the search by instructing the search engine to search for all the records containing the first keyword, then for all the records containing the second keyword, and show only those records that contain both.
In CLIO, if you enter search terms without an operator, AND will automatically be inserted between them (but make sure that Javascript is enabled on your computer, or the search will fail).

The operator OR broadens the search to include records containing either keyword, or both.
The OR search is particularly useful when there are several common synonyms for a concept, or variant spellings of a word.


  • adolescent or teen?
  • medieval or "middle ages"
  • vergil or virgil

Combining search terms with the NOT operator narrows the search by excluding unwanted terms.

These Venn diagrams help to visualize the meaning of AND, OR and NOT; the colored area indicates the items that will be retrieved in each case.

moths AND butterflies

venn and

You want to find books that are about both moths AND butterflies

moths OR butterflies

venn or

You want to find books that are about moths, books that are about butterflies, and books that discuss both:
OR means MORE.

moths NOT butterflies

venn not

You want to find books that are about moths, but EXCLUDE those that discuss butterflies

If you would like further assistance, Ask a Librarian for help.