Marissa Mayer, vice president for product development at Google, recalled concerns raised during internal discussion about the likelihood of encountering advertiser resistance to such an unfamiliar format. At one point near the time of the debut, one of her colleagues leaned over and predicted, "You wait, in a month we'll be selling banners."

It did take a little while before prospective sponsors were willing to try Google's text ads, but soon enough, they attracted the intrepid. Mr. Brin and Mr. Page deliberately offered advertisers instant gratification: pull out your credit card, plunk down a $50 deposit, send in four lines -- and in a blink it would be out there, having been automatically processed without a pre-publication review by a humanoid. (Google's language police would follow up later, if need be.)

Ms. Mayer credits small companies for helping to draw the attention, and ad dollars, of Google's big accounts. Because of the sheer number of commercial sites run by small operators -- like the one that has bought a sponsored link tied to the unappreciated sport of extreme ironing -- their customers add up to a very large number.

Once upon a time, Goto, another pioneer in online ads that was renamed Overture and bought by Yahoo, thought that search result positions should be sold to the highest bidder. Bad idea. Users wanted the order of results determined by algorithm, unswayed by advertisers. That wish became the unwritten law for search. Today, Yahoo and MSN serve up text-only ads in the same peripheral locations on the page as Google, and use an almost identical format. Like Google, they also are fighting the good fight against pop-ups, and forbid advertisers from linking to pages that will bop the user in the nose. Google's model is copied for a simple reason: its ads produce profits that prove that size does not matter.

Some analysts view Google's embrace of text as temporary, predicting that the company will add image advertising to its site just as soon as it can build the infrastructure. Jordan Rohan, an Internet analyst at RBC Capital Markets, said that given the fact that Google serves up 100 million search-query results a day, were the company to add a single photo-quality image to each page, the bits for each page would increase a thousandfold and the resulting load would figuratively "break the Internet."

Is Google holding off on image ads because of inadequate infrastructure? No, responds Ms. Mayer. She says Google uses text for ads because of cognitive science: text has the highest information density and allows users to scan a lot of information at the highest speed, or, as she phrased it, "the bit rate on text is very fast." Anything that gets in the way of speed-reading must go. Google does not permit advertisers to use all-capital letters. (Studies have shown we read those 30 percent slower than properly capitalized words.) Ms. Mayer did allow for one theoretical exception to text ads in the future: when users search for videos. "For a query on videos for baking a cake, then, a video might be best," she said.

The online marketing agency Avenue A, which later became Avenue A/Razorfish, says that about 30 percent of the more than $400 million it will spend on behalf of its clients this year will be for text ads on search pages. Last month, Eric Schmidt, Google's C.E.O., said the company's profits jumped sevenfold in the third quarter, versus the period a year earlier, partly because larger companies were increasingly willing to spend their ad dollars on search-related advertising.