NOT is used to narrow your search. When you add NOT between two or more search terms, your search results will include only the term before the NOT.
The blue area in the illustration to the right represents the results that would be returned for the search Cetacean NOT Whale. All the articles will have only one of the terms - Cetacean.
Lets take a closer look.
In the database, Web of Science, the searches for Cetacean, Whale, and Cetacean NOT Whale break down like this:
Cetacean = 6,652 results
Whale = 19,749 results
Cetacean NOT Whale = 3,186 results
Follow-Up Question: Do you think Whale NOT Cetacean will give the same number?
It is important to remember that NOT is very powerful - if a record includes just one instance of your "notted" term, it will be excluded from your search results, even if it is relevant.
Take the example to the left, Cetacean NOT Whale. Whales, dolphins and porpoises are all marine mammals, or Cetaceans. If you wanted to research only dolphins and porpoises, this search would exclude articles about Killer Whales - which are actually a part of the scientific family that includes dolphins and porpoises.