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A vast majority of the web (90%!) cannot be accessed via search engines like Google. From Mashable.
Keywords vs. Subject Headings
KEYWORDS are words you come up with on your own, using the main concepts of your research question. Create synonyms of these concepts to create more. For example for "shrimp farm," you might also try "aquaculture" or "shellfish farm." Other examples:
lysistrata AND "sex strike"
"stonewall Riots" AND "gay rights" OR law*
"arab spring" and influence
"tea party movement" AND influence
SUBJECT HEADINGS are vocabulary a librarian assigns to different records in the catalog. You can find subject headings by finding an article that is relevant and looking at the subject headings that have been assigned to it. Use one or more to search via a keyword or subject heading search.