What is a pop-under ad?

A pop-under ad is an online advertising format where a new browser window or tab opens behind the user’s active window rather than on top. The user notices the ad later — when closing the main tab or switching windows. Described alongside pop-ups as a variation: where pop-ups interrupt immediately, pop-unders defer contact.

History and mechanics

Pop-up advertising emerged in the late 1990s on hosts like Tripod.com when JavaScript enabled extra windows. Developer Ethan Zuckerman later framed pop-ups as a compromise between advertisers and content placement; pop-unders evolved as a less intrusive but more covert alternative.

Technically, pop-unders use window.open() followed by window.focus() on the parent window so the new tab stays behind. Modern browsers restrict automatic window opening without user gestures, so networks bind calls to page clicks.

Difference from pop-up

Factor	Pop-up	Pop-under
Position	Over content	Behind active window
Immediacy	High, interrupts UX	Deferred
CTR	Often higher when aggressive	Lower, but cheaper CPM
Blocking	Aggressive blockers	Partially evaded, declining

Pop networks and arbitrage

Pop-unders underpin pop traffic: buying through specialized pop ad networks on CPM or CPC. Traffic is often cheaper than social or search, but CR depends heavily on vertical, GEO, and placement quality. Common funnels: gambling, dating, antivirus, sweepstakes, subscription offers.

Buyers filter zones (whitelist/blacklist), test frequency caps, and compare ROI by placement sub_id in the tracker.

Quality and reputation

Pop-unders associate with aggressive monetization and low user trust. Some advertisers and networks ban pop traffic in offer terms. Malware and auto-redirect placements trigger antifraud filters in trackers and networks.