Pop-under ads are a slightly more polite version of a pop-up.
They still appear in a new window, but the window is minimized.
Often the user doesn't notice it until they close the main browser
window.
Pop-unders generally annoy users less than pop-ups, but when several
of them pile up they can still cause trouble. I recommend displaying
only one pop-under per user per day or per browser session.
Unfortunately, pop-unders are often lumped into the same category
as pop-ups, and most pop-up blocker software will also prevent
pop-unders from displaying. As the use of these programs increases,
revenue from both types of pop ads will decrease.
