Gardiner, Alan Henderson (1957), Egyptian grammar being an introduction to the study of hieroglyphs, 3rd, revised edition, page 515
Fischer, Henry G. (1977), The evolution of composite hieroglyphs in ancient Egypt. Metropolitan Museum Journal 12, 5-19, p. 7
Lacau, Pierre (1954), Sur le système hiéroglyphique. Bibliothèque d'étude 25, p. 19