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.
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.