Bargain travelers will be able to book trips for less than $100 round trip this winter.

But, to do so, they'll have book by Thursday (Oct. 11) and fly on short routes close to home. They also must be able to stick to fare-sale fine print that includes black-out dates, promo codes and a few route restrictions. 

Southwest Airlines and subsidiary AirTran have each rolled out nationwide sales that drop round-trip airfares to less than $80 on their shortest routes.

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