Boolean Operators let you combine or exclude terms, or search related terms. The three main Boolean operators, common to almost every database, are AND, OR, and NOT. Here is how they function:

(a) The and operator. When search terms are connected by and, all of the terms will appear in the results. Results for a search of cats and dogs will return only results that have to do with both cats and dogs (and possibly hamsters, ferrets, etc.). Material that discusses cats but makes no mention of dogs will not show up in the record list. 

 (b) The or operator. Or is used when you want to return results for related or synonymous concepts. It lets the database know you want results that contain any of your terms, regardless of whether or not they appear in the same document. There is no limit to the number of terms you can link with or, but bear in mind that more terms yield more results, which is not always desirable. The object is to find the right results, so use or sparingly.

(c) The not operator. Linking terms with not excludes a term from a search. Cats not Siamese will eliminate any mention of Siamese cats from the results. Be aware, though, that not might eliminate useful articles that merely mention your terms. It is usually better to include the proper keywords in an and search.

Truncation and Wildcards. Truncation allows you to retrieve multiple forms of a word by entering the stem of the word into the search box and adding that database’s truncation symbol. To find write, writer, written, or writing, enter the letters common to all forms, writ, and add the truncation symbol (e.g., writ*). Frequently the truncation symbol is an asterisk (*), though it might be a different character. A database usually has search tips or a help page explaining its symbols.

Wildcards work much like truncation, but instead of adding to the first few letters of a word, they can substitute for one or more characters. For example, if the wildcard symbol is a question mark (?), typing wom?n will return results containing the word woman or women. You can also use a wildcard to pick up variations in spelling: reali?e will find both realize (American spelling) and realise (British spelling).

Phrase Searching. When you want to ensure that words remain together in a particular order, use phrase searching. Most often, phrase searching is accomplished by placing the search terms in quotation marks. “Comedy of manners” will produce records where those three words appear as a phrase.

Nesting and Search Strings. Sometimes searching with Boolean operators can be ambiguous. Your search may consist of a string of terms connected with and, or, and not. The database may have an established order in which it looks at terms, but that won’t always be evident to the researcher. To ensure that your search is understood by the computer, you can use nesting, a technique by which you can group operations using parentheses. The nesting parentheses in a search string work much the same way as they do in mathematical equations. The result of 5 + 12 ÷7 - 4 is different from that of (5 + 12) ÷ (7 - 4). We can make the database understand what we want by using nesting. Like Ukranian Matryoshka dolls, nesting places one thing—in this case, one search statement—within another. To control how your search terms are understood, place parentheses around the elements: "german*" and (expressionism or modern*) and (drama or theat*).

George Boole (1815-1864), the English logician who gave us the Boolean Operator.