Boolean operators are words you can use to connect your keywords in your searches. AND, OR, and NOT are types of Boolean operators.
You can connect keywords with the word AND to narrow your search. AND refines your search by looking for the occurrence of multiple search terms in a source.
Example: "Black Death" AND culture AND economics
You can connect keywords with the word OR to expand your search. OR expands your search by finding the occurrence of one search term or another. Keywords combined with OR should be enclosed in parentheses.
Example: ("Black Death" OR "Black Plague")
You can connect keywords with the word NOT to limit your search. NOT limits your search by finding one search term while omitting another.
Example: "Bubonic Plague" AND (Europe NOT China)