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See the headline below:

What the traveler needs to know about the Euro

http://goeurope.about.com/cs/currency/a/the_euro.htm

I think "the traveler" should be "a traveler".

Right?
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It can be either "a traveler" or "the traveler" or "travelers (need)".

The use of the definite article "the" is certainly a peculiar substitute for the indefinite "a", but there are certain conditions under which this substitution can be made.

Unfortunately, I don't which conditions those are! Emotion: crying I'm going to think about it some more and get back to you if I manage to work out something resembling an explanation!
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A, an and the can be used to specify that a noun refers to the whole class to which individual countable nouns belong. We call the usage of articles in such manner generic.

What a traveller needs to know about the Euro. (any traveller, not specific)

What the traveller needs to know about the Euro. (all travellers)
I've learned another property of the!