Boolean searching is based on an algebraic system of logic formulated by George Boole, a 19th century English mathematician.
In a Boolean keyword search, the terms are combined by the operators AND, OR and NOT to narrow or broaden the search (in CLIO, Ovid, and some other databases, you DO have to enter them in capitals). This type of search is possible in most library catalogs and databases, but Google and other Web search engines do not carry out OR and NOT searches properly.
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.
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.
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 using OR:
medieval OR "middle ages"
"heart attack" OR "myocardial infarction"
vergil OR virgil
Combining search terms with the NOT operator narrows the search by excluding unwanted terms.
Examples using combinations of the three operators:
puritans AND women AND (massachusetts OR connecticut OR "rhode island" OR "new hampshire")
(adolescen* OR teen*) AND (cigarettes OR smok*)
reagan AND "star wars" NOT (movie OR film OR cinema OR "motion picture")
"zora neale hurston" AND (correspondence OR letter* OR diar* OR autobiograph* OR memoir*)