Following a previous announcement by Google last November that they were starting to add 'Sitelinks' to the AdWords options, their blog has now confirmed that this feature is being rolled out to all advertisers.

Google says that the initial test has shown advertisers seeing huge gains with Ad Sitelinks, driving clickthrough rates up by more than 30% on average (and of course boosting Google's revenues at the same time!). The Ad Sitelinks option is now available within the campaign settings for all accounts and enables advertisers to create more links to relevant content pages from the AdWords listing.

Therefore when a user's search query matches a keyword in an Ad Sitelinks-enabled campaign, Google will automatically determine if the advert qualifies to show Ad Sitelinks and whether to show the two-line or the one-line format based on the quality of that ad. Since this feature is a campaign-level extension, the links that are created by the advertiser should be relevant to any advert in that campaign.