It is essential to understand Boolean logic to search databases and the Internet effectively. Boolean language deals with sets and with which items meet named criteria. It can be deceptively hard because of the simplicity of the words used: AND and OR. The explanations below reveal how the Boolean use of these words differs from the natural language use of them.

1. Using OR between words says to the database that you will accept any item that meets either criterion.
2. OR is useful for separating words that represent the same general idea or concept.
3. The resulting set will be larger (broader) than the set from a single word search.
4. How it differs from natural language: The word "or" often means that one choice of several--narrows the field rather than broadening it as in Boolean language.

Visual example:
For each search phrase, how many items meet the criteria (fit into the set)? Q1. Yellow OR green Q2. fish OR bird Q3. lion OR tiger		a. b. c. d. e. f.
Answers: # in set   Q1. 4 (specifically, a, c, e, & f) Q2. 2 (specifically, c & e) Q3. 1 (specifically, only b)

Verbal example:

• Using AND between words says to the database that you will accept only those items that meet both criteria simultaneously.
• AND is useful for combining concepts, for finding where two or more concepts intersect.
• The resulting set will be smaller (narrower) than the set from a single word search.
• How it differs from natural language: The word "and" often means an increase in number--it adds to a group rather than requires an additional feature as in Boolean language.

Visual example:
For each search phrase, how many items meet the criteria (fit into the set)? Q4. Yellow AND can fly Q5. black AND has legs Q6. lion AND tiger		a. b. c. d. e. f.
Answers: # in set   Q4. 2 (specifically, a & c) Q5. 4 (specifically, a, b, c, & d) Q6. 0 nothing can be a tiger and a lion simultaneously!

Verbal example:

• Using NOT between words says to the database that you want everything that meets the first criterion as long as the second criterion is not present.
• NOT is useful for excluding items, particularly when the first word has multiple contexts.
• The resulting set will be smaller than the single word search.
• How it differs from natural language: It doesn't! This is the easy one.

Visual example:
For each search phrase, how many items meet the criteria (fit into the set)? Q7. water-dweller NOT yellow Q8. black NOT insect Q9. animal NOT lion		a. b. c. d. e. f.
Answers: # in set   Q7. 1 (specifically, e) Q8. 2 (specifically, b & c) Q9. 6 since there is not a lion in the full set, NOT eliminates nothing.

Verbal example:

• You can use multiple Boolean operators in a single search
• The operators do not act in order from left to right; instead, the AND operates first, then the OR, and finally the NOT.
• To override the order of the operations, use parentheses to nest terms together.

Visual example:
For each search phrase, how many items meet the criteria (fit into the set)? Q10. insect OR bird AND yellow Q11. (insect OR bird) AND yellow Q12. (yellow OR black) NOT (insect OR bird)		a. b. c. d. e. f.
Answers: # in set   Q10. 3 (specifically, a, c, & d) Q11. 2 (specifically, a & c) Q12. 2 (specifically, b & f)

Verbal example: