When Yahoo announced last week that it had begun allowing sites to pay for inclusion in the Web index it uses to generate search results, it drew criticism for blurring the line between advertising and legitimate search results. But Yahoo's approach is not unprecedented. At least two other sites, MSN Search and Ask Jeeves, allow companies to pay to be indexed. (Ask Jeeves says it will phase out paid inclusion in April.) According to Yahoo, the practice will not corrupt search results. As the Web keeps growing, billions of Web pages are overlooked. Inclusion in the index means that the site will get searched, not that its relevancy will be artificially inflated, says Yahoo vice president Tim Cadogan. Rankings are determined by relevancy algorithms and link analysis, he says, adding, "There is a total separation of church and state." To learn more about how search engines work, visit searchenginewatch.com.