SQL Server always allocates space for objects in units of eight contiguous pages called extents. All pages in the extent are owned by the same object. When SQL Server needs to give more pages to an object, the server designates a new extent for that object. If all the rows in an extent are deleted, SQL Server frees the extent for use by any other object needing space. All of the extents for an object are chained together in a doubly linked list called an extent chain. An extent chain can span multiple allocation units.