priceline is probably the better known, thanks to its extensive promotion program featuring William Shatner. Priceline's shtick is "Name your own price," which means that you enter the amount you're willing to pay for a flight from one city to another, a hotel room in a specific city in specific "star" range, or a specific size class of rental car. For a hotel room, if you bid enough to cover whatever minimum price the site has negotiated with any hotel in the class you specified, you get the room at that price. Priceline then tells you the name of the hotel. If your bid is too low, you can vary the bidding conditions a bit and try again. Priceline covers mainly hotels in the United States, with some major overseas coverage; affiliates priceline.co.uk and bookings.com provide much greater coverage in Europe.

Hotwire tells you, in advance, how much you'll pay for the hotel room, but not which hotel. Once you make your choice, Hotwire's system works like Priceline's. Hotwire's coverage outside North America seems scantier than Priceline's, and I haven't seen any comparable overseas affiliates.

Priceline has expanded into fixed-price travel services - in fact, these days it seems to be emulating Expedia, Orbitz and Travelocity more than concentrating on its opaque offerings. Hotwire hasn't added fully disclosed prices; instead it links to its affiliated site, Expedia.

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