Used for lightweight, but very large things, like rocket/missile bodies - or for Boeing, flying parts of their unassembled airplanes around to the final assembly point.

A modified 747 (the Dreamlifter) holds fuselage sections of the new 787 Dreamliner inside. Yeah, that's globalization. http://images.google.com/imgres?imgurl=http://boeingblogs.com/randy/images/Alenia_Dreamlifter_ip.jpg&imgrefurl=http://boeingblogs.com/randy/archives/2007/03/&usg=__jKq6_40D-ZYINBaAeoJLADBLf8E=&h=307&w=480&sz=61&hl=en&start=7&um=1&tbnid=aApHCEhjto_Q7M:&tbnh=83&tbnw=129&prev=/images%3Fq%3Ddreamlifter%26hl%3Den%26rls%3Dcom.microsoft:en-us%26um%3D1