Piggybacking is a technique used in the context of online advertising to enable an ad server to serve an ad from a different source or network if it is unable to serve an ad from its own inventory. Piggybacking is similar to passbacks, but it involves the use of multiple ad servers or networks, rather than a single ad server or network. Piggybacking can be beneficial for advertisers, as it can help to ensure that their ads are delivered to users in a timely and efficient manner. Piggybacking can also be beneficial for publishers and platforms, as it can help to maximize the value and revenue of their ad inventory, and to provide a better user experience for their audience.