Found an apparent incongruity:
"Control information typically takes one of two forms: headers and trailers. Headers are prepended to data that has been passed down from upper layers. Trailers are appended to data that has been passed down from upper layers. An OSI layer is not required to attach a header or a trailer to data from upper layers."
Found about 60-70 rows down in the webpage entitled: http://docwiki.cisco.com/wiki/Internetworking_Basics