Pop-under ads are browser windows that are opened underneath the browser window, which means that they are not directly seen and then closed shortly thereafter.
Pop-under ads are similar to pop-up ads in that they open in a separate browser window; the difference is that they are hidden underneath the current browser window. These adverts are therefore not noticeable at first, but they are a source of annoyance to users later on.
From an advertiser’s point of view, pop-unders may sound like a great idea; after all, the user is not interrupted in his current action. However, this is not a view that is shared by web users, pop-up blockers and web browsers. Many pop-under ads are prevented from opening in Chrome, for example.
Google itself states that the use of pop-under ads is not allowed, on the grounds that these ads are not conducive to a good user experience and are just annoying.
Pop-under ads are not even included in the advertising standards of the “Coalition for Better Ads”.