In general, one aspect of the subject matter described in this specification can be implemented on a computer system. A quantity of slots associated with one or more publications can be received from/identified by a publisher. Slot properties for each of the slots in the received quantity of slots can be determined including determining the permitted expansion directions for the slots based on the slot properties. Content items can be selected for publishing from one or more expandable content items based, at least in part, on the possible expansion directions of the slots.