Sponsored search (also called sponsored links or search ads) allows advertisers to be included in the sponsored results of a search for selected keywords. Search ads are often sold via real-time auctions, where advertisers bid on keywords. In addition to setting a maximum price per keyword, bids may include time, language, geographical, and other constraints. Search engines originally sold listings in order of highest bids. Modern search engines rank sponsored listings based on a combination of bid price, expected click-through rate, keyword relevancy, and site quality.