As you widen your search, you may well be aware of subject matter which your search will retrieve, but which you know you don't want. An example might be research into angels where you know you don't need information about the California Angels baseball team. To exclude this information, use the NOT operator to eliminate documents with words which might indicate the baseball team. NOT Anaheim NOT baseball. BUT NOT baseball.
Some search engines have their own specialized syntax and some have additional operators.
These operators are called Proximity Operators to distinguish them from true Boolean Operators. Let's look at some other techniques which can affect deep searches. To narrow a search to a usable number of documents, you might want to restrict a search to titles or headings on the grounds that this will limit the search to documents which include your search parameter as a main topic. Another technique is truncation, although this one can backfire on you. For those engines which assume truncation, you may wish to turn truncation off, when typing a full word, by ending the word with a space.