Boolean operators (AND, OR, NOT) serve to either narrow or expand your search:
OR expands it by including synonyms or related terms:
AND narrows it by finding the subset of articles that contain both search terms you want:
NOT narrows your search by eliminating articles that contain a specific term you do not want:
An easy way to remember the difference is the rhyme OR gives you MORE, while the other two operators give you less (although there may be better results).
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