1. A method for displaying expandable content comprising: maintaining, by a data processing system, a content database including a plurality of entries, each entry including a respective slot identifier identifying a corresponding content slot and a respective resource identifier identifying a corresponding electronic publication on which the respective content slot is positioned; receiving, by the data processing system, a request to provide content, the request identifying a slot identifier corresponding to a content slot in which to provide the content and a resource identifier identifying an electronic publication on which the content slot is positioned; determining, by the data processing system, by accessing the content database, one or more permitted expansion directions for the content slot using the slot identifier and the resource identifier; and selecting, by the data processing system, an expandable content item for publishing on the content slot of the electronic publication based on the one or more determined permitted expansion directions.