A pop-under is an alternative to a pop-up and is an advertisement which opens in a new browser window, initially hidden under the active window. Pop-unders are less intrusive than pop-ups, as they don’t interrupt the viewer, but they are also sneakier, as it’s more difficult for the viewer to remember which website opened that pop-under, if browsing through several pages. Technically, pop-ups and pop-unders are similar, but the latter is considered less disturbing by users and is effective for paid advertisements.