Submitted by Dyske on May 23, 2003

Dual Purpose or Dual Purposes?

“This knife has dual purpose.” Do I need to pluralize “purpose”? After all, the statement is saying that it has more than one purpose, namely two purposes.

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The correct way to say it is: "This knife has a dual purpose." You could also say, "This is a dual-purpose knife.

It could be possible that the knife has dual purposes, but that implies that the knife has more than one dual purpose. I suppose in that case one is more apt to say, "This is a multi-purpose knife."

To be specific:

Dual has two main definitions
1. Having related/similar parts
In this case you often would use the plural.
"My computer has dual processors."

2. Having a double purpose or role
Here you almost use the singular; a dual purpose is a double purpose purpose. :)
eg dual citizenship, dual nature, dual truth

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Perhaps my response seemed to exclude Rufus's and adaiha's examples as correct. They are both correct; I just went on to iterate what you actually mean if you use the plural. That is, what someone with perhaps a more logical bent would think you mean. ...Though it appears that esc has something against logic and reducing to an absurdity :)

BTW, I would tend to consider a dual purpose knife to be just a underachieving multipurpose knife :)

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"This knife is dual-purpose."

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"This knife has dual purposes," is also acceptable.

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Rufus is right. Vid: This knife has dual purposes. This knife is dual purpose.
A knife of a different color. While potentially correct and illuminating IngisKahn's answer is reductio ad absurdam, and multipurpose knives are not under discussion. American English is a living language and what feels correct is generally acceptable, as long as subject, verb and object are in some semblence of harmony. I won't discuss British English.

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Although I suppose this is merely dodging the answer and doesn't really answer your question, if you are really perplexed, you can also say this:

"This knife has two purposes."

Again, I agree with IngisKahn. I personally consider "the knife has a dual purpose" the most accurate answer. I believe "dual" is an adjective describing a singular entity with two parts. It's like using the noun "couple." For example, think of "a couple of lovebirds." The noun "couple" refers to one and only one set of two lovebirds.

I don't think "dual purposes" is necessarily wrong, but I don't think it fits as well for your example. I think "dual purposes" would be more accurate when in the subject of the sentence, rather than the predicate. Example: "the dual purposes of my computer are to communicate with others and play video games."

Also, I think saying "this knife is dual-purpose" sounds like you are trying to sell a knife. It describes how many things it can do, rather than why it exists.

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The knife has two purposes. It is dual-purpose. An object having "dual purposes" is awkward English.

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