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What is Boolean searching?

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

In Boolean searching, keywords are combined with the operators AND, OR and NOT to narrow or broaden a search (you do not have to enter them in capitals).  Here’s a great video that explains using Boolean searching.

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

AND
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.  

OR
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.

Examples:
 adolescent or teen?
 medieval or "middle ages"
 vergil or virgil

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