Boolean operators help expand or narrow the scope of a search. A search for rivers OR lakes returns documents with either word in them. A search for rivers AND lakes returns documents with both words in them. A search for rivers AND lakes NOT swamps returns documents that mention both rivers and lakes but omits those that also mention swamps. Implied Boolean Operators are characters such as and , which can be used to require or prohibit a word or phrase as part of a search expression. The acts somewhat like AND, and the acts as NOT would in a Boolean expression. For example, the Boolean expression rivers AND lakes NOT swamps may be expressed as rivers lakes swamps.