Associated Newspapers' complaint centres on the way that the highest bidder is instantly selected for advertising space every time someone accesses a page. Google's dominant DoubleClick Ad Exchange (AdX) system allows advertisers to bid to put ads on web pages. MailOnline has been using "client-side header bidding", where publishers can attract outside bids, increasing competition for advertising space. Google "punished" the website "because Daily Mail's pages were less profitable to Google than other websites," the legal complaint claims. "For years, Google substantially has eroded Daily Mail's search traffic," it claims, accusing Google of using its "monopoly search engine to entrench its ad-tech dominance". It said the problem "persists to this day", adding that since the beginning of the year, Daily Mail's US search traffic has fallen more than 50 percent.