A tautology is an expression or phrase that says the same thing twice, just in a different way. For this reason, a tautology is usually undesirable, as it can make you sound wordier than you need to be, and make you appear foolish. Occasionally, a tautology can help to add emphasis or clarity, or introduce intentional ambiguity, but in most cases it's best to choose just one way to state your meaning and eliminate the extra verbiage.
Sometimes a tautology involves just a few words that mean the same thing. Consider the following sentence:
This is an example of tautology, because the adverb "personally" repeats the idea already expressed in the single word "I". In everyday conversation, the addition of "personally" is used for emphasis to point out that the subject of the sentence is invested in the action, has overseen something, etc. Technically, the word "personally" doesn't add any new information and could be cut from the sentence without changing its meaning.
In the realm of logic, a tautology is something that is true in all circumstances. A common example of a logical tautology is the following:
This sentence is always true because one or the other must be so. This is different than a statement that says, "The dog is either brown, or the dog is white," because dogs can be black, gray, or a mix of colors. Note that when you put both halves of the logical tautology together, it feels a bit redundant, just like a verbal tautology.
A tautology often involves just a few words in a sentence that have the same meaning, or in which one word is part of the definition of the other word. Though tautologies are common in everyday speech and don't diminish clarity, they should be avoided in formal writing so you don't repeat yourself unnecessarily.
The highlighted words in these examples are tautological; that is, they have similar meanings. This is no need to use both:
Even the best speakers and writers will sometimes let tautology slip into their work. Politicians are particularly prone to this verbal tic as they speak for hours on end and often give slightly different versions of prepared remarks during many campaign stops.
Occasionally tautologies are more than just needless repetition; they can add beauty or cause the reader to think about a subject more deeply. Examples of tautologies in literature show them at their best, whether for dramatic or comedic effect:
Song lyrics are often a treasure trove of tautologies, as in this case repetition can be artistic. Additional words can help fill out the rhythm or make a rhyme in song, so this repetition is often in service of the artistic work as a whole rather than accidental:
Sometimes there is tautology with the use of abbreviations and acronyms, when part of the acronym that stands for a word is then repeated in conversation. For example, saying "the ATM machine" is a tautology, because the M already stands for machine. Other examples include:
Repetition is often a key feature in advertising. Marketers want to make their messages stick in people's minds to encourage them to buy. For this reason, tautology is common in store signs and advertising slogans:
In a logical tautology, the statement is always true because one half of the "or" construction must be so:
As you can see, there are times when the use of tautology is helpful to emphasize a point or to add poetic flair to speech or writing. In most cases, though a tautology doesn't add new information to a statement and should be edited out of your writing. This will show readers that you are in complete command of your vocabulary and don't need to use extra words to make your point. In writing, as in life, sometimes less is more.