These three little words (celled “Boolean Operators” for mathematician George Boole, 1815-1864) allow you to control how a database search engine will combine the terms and ideas that define your research topic.

They are a basic building block of all library and Internet search engines, and learning how to use them will give you a great deal of control over your search, and improve the quality of your results.

The following diagrams explain how each operator works.

AND narrows

Finds only the records that contain both keywords.

a AND b


Use AND between keywords that represent different ideas.

cats AND allergies
OR expands

Finds all the records that contain either keyword.

a OR b


Use OR between keywords that represent the same idea.

domestic OR household
NOT excludes

Records must contain the first term and exclude the second.

a NOT b


Use NOT to exclude an irrelevant keyword from your results.

pets NOT fish

You may notice that most Advanced Search pages are designed to help you group (or nest) your keywords to produce more complex (and efficient) search statements.