The Mobile Portrait Interstitial Ad (320 x 480) is a huge revenue driver for businesses whose traffic majorly originates from mobile. It is also called mobile interstitial - a full-screen ad on smartphones that appears between two pages. It falls under the category of expandable ad, and even though it is out of date, it is still used widely and can also function as a mobile video ad. The ad can be very annoying to users, so use it sparingly.