Partnerships between the top search engines and directories.

There has been much written about the inability of any search engine or directory to cover the whole of the web, leaving millions of websites and pages unlisted or 'invisible'. In an effort to increase their coverage of websites and to generate additional revenue from their services - as well as to build user loyalty - many of the top search engines and directories have created alliances with each other, as shown of the table below.

This factor can help the search engine marketing of websites, because even if a site does not get listed by one search tool, there is a chance that the secondary results will still help it to appear in the search listing.

The following table shows the partnerships between the main US search engines and directories, indicating which tools provide or receive search results from other tools. You can also view the partnerships between UK-specific search engines and directories.

* Results from pay-per-click search tools Google AdWords and Overture are included as 'sponsored results'.