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If you enter Vietnam War as your search terms, you wil get any item that incudes the word Vietnam AND War.
If you enter "Vietnam War" as your search term, the search engine will only retrieve items with those words in that exact order. This can greatly increase the relevancy of your search results and will help you avoid sorting through long lists of potentially-irrelevant results.
Using an Asterisk
If you enter the term soldier as your search term, you will only get results that contain this exact word.
If you enter the term soldier* as your search term, the search engine will retrieve items that contain any spelling variation of this word (for example: soldier, soldiers, soldiering, etc.). This can help to expand the number of search results that you get and will make sure that you are not limiting your search to a specific spelling of a keyword.