The three most common Boolean operators are the words AND, OR, and NOT, which can be used to connect your search terms in order to expand or narrow your search. For example:
Connecting terms with OR means you'll accept results containing any of the connected terms and therefore broadens your search (think "OR means more"). A search for cars OR trucks brings up all results containing only the term cars and all those containing trucks, plus all those that contain both cars and trucks. This operator can be useful to use with synonyms so you catch all relevant articles, even though they may use slightly different terms. For example, cars OR automobiles. Or phrases should be encased in parentheses.
Connecting terms with AND means you want all of those terms to appear in your search results, which narrows your search. A search for cars AND trucks brings up only results containing both of those terms.
Adding NOT to your search excludes terms that you don't want in your results, and so narrows and focuses your search. For example, cars NOT trucks. This can be useful if you are getting a lot of irrelevant results because a search term has more than one meaning.
You can combine Boolean operators to create a more precise search, such as (cars OR automobiles) NOT trucks AND ("gas mileage" OR "miles per gallon" OR mpg). Here's how it would look in a database search, such as Academic Search Premier:
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