Kayak, the popular multi-airline airfare search engine, thinks Microsoft Bing's new travel search engine looks so much like its own that it's confusing Kayak users. The travel search company sent Microsoft a legal letter last week telling them to cut it out, Wired.com has learned.

Microsoft heralded its travel search as one of the key ways that its revamped search engine Bing bested Google by helping users make decisions, rather than just finding information.

Its search results for an itinerary presents users with sliders and check boxes on the left that let searchers change times and specify airlines. Search results reload instantly as boxes are clicked and sliders slid.

There's no question Bing feels like Kayak. When Microsoft showed us the search engine under embargo, this reporter's first comment upon seeing the travel page demo'd was "This looks like Kayak." Our Bing review described its interface as "uncomfortably close to Kayak's," an observation that others made as well.

Kayak noticed too.

"We have contacted them through official channels about concerns about the similarities between Bing and Kayak," Kayak's chief marketing officer Robert Birge told Wired.com "From the look and feel of their travel product, they seem to agree with our approach to the market."

That's careful language for "Microsoft copied our stuff wholesale."

Microsoft's Whitney Burk denies that there's any copying going on.

"We are discussing the matter with Kayak," Burk said in emailed statement. "Bing Travel is based on independent development by Microsoft and Farecast.com, which Microsoft acquired in 2008. Any contrary allegations are without merit."

Copyright law offers some protection for a website's look-and-feel, but it's not easy to prove.

Others noticed as well.

For instance, the Pittsburgh Post-Gazette's David Radin reviewed Bing, noting that the travel site "feels so much like Kayak that without asking, I assumed Microsoft licensed the technology from Kayak. Can you say 'eerily similar'?"

Douglas Sims, the president of an IT consulting firm in Tennessee, noted the similarity too, inspiring him to write a short essay on his website:

Bing Travel is like Kayak in more ways than just the layout and visual design. The navigation, the automatic loading of results from different airlines (using ajax), even the results themselves are almost identical.

In an interview, Sims said he had no connection to Kayak but was moved to write about the comparison because he was a longtime admirer of Kayak's interface. In early June, he had friends writing to tell him how cool Microsoft's search was.

"I felt offended," Sims said. "I thought Kayak did an awesome job, and now my friends are giving credit to someone else."

He noted that Microsoft's travel search engine includes the same shading on the gray sliding bars as Kayak's site does.

"I design stuff like this sometimes," Sims said. "I've always said I can't just copy other people's work, you can't just rip it off, but in hindsight, is it fair game? Maybe I should do that."

Readers at TechCrunch also had a healthy debate about whether Microsoft ripped Kayak off, or was just using the technology it bought when it purchased Farecast.

For its part, Kayak seems caught between indignation at seeing a giant company copy their hard work and flattery that their work at designing a user interface work is so good that someone would ape it.

They are also a company with fewer than 100 employees, while Microsoft employs far more lawyers than that.

Update: This post was updated Wednesday evening with comment from Microsoft.

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