Connectors (Boolean operators) are used in keyword searches to link together two or more terms. The most commonly-used connectors are: AND and OR.

AND narrows a search; you get fewer records, because both words must be present in the records found -- here the dark area in blue. Use AND to combine different concepts in one search.

computer AND history AND mathematics

OR broadens a search; you get more records because OR includes each of the words separately, as well as all words when found together. OR is often used to link together related words.

computer OR history OR mathematics