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The pop-under describes a modified form of the pop-up window. In contrast to the pop-up that pops up in the foreground, the pop-under is opened by the web browser in the background so that it is often not visible at first. The pop-under is usually only noticed when the currently viewed page is closed again.

Often the pop-under effect is simply based on the fact that the website actually accessed jumps to the foreground and thus covers the pop-under. Due to simply functioning pop-up blockers and a decrease in opposition, the pop-under is now experiencing a renaissance. Simple pop-unders are, however, also recorded and blocked by pop-up blockers.

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