The adCenter blog have added an extra ad placement opportunity with the addition of a fourth ad in the mainline above the organic results. (Mainline ads are the ads that appear above algorithmic results, and right-rail ads are those that appear to the right of the algorithmic result set.) This increases the ads shown from 8 to 9 per page, which means there is an additional ad placement opportunity for you to vie for in the adCenter auction. In addition, the top two mainline ads will continue to receive essentially a two-for-one ad placement, since they will also display at the bottom of the page in the U.S.