Google, Yahoo!, and other search engines offer advertising spots in their results pages that look similar to regular listings. These spots appear in the top, right, and bottom margins of search engine results pages. Pay-per-click ads look similar to "natural" listings - the unpaid listings in the middle of search engine results pages. The main difference between "natural" listings and paid listings is advertisers pay a specific amount whenever a user clicks on a paid listing's link (hence the name "pay-per-click").