Feather keratin has a composite structure with a filament-matrix texture, and transmission electron microscopy studies of thin transverse sections of feather rachis by Rogers and Filshie in the early 1960s showed that the filaments have a strong tendency to form sheets. Potentially this could account for the unusual X-ray diffraction pattern noted by Bear and Rugo in the early 19505, which was interpreted by them as indicating a two-dimensional net structure. Although it is 50 years since these major advances were made the possibility of extracting information on the nature of the filament packing from the diffraction pattern has never been explored. The present contribution shows how, when taken together with current information on the nature of beta-sheets in feather keratin, certain features of the X-ray diffraction pattern can now be used to determine the likely arrangement of the filaments in the sheet. (C) 2011 Elsevier Inc. All rights reserved.