Borghouts, J. F. (2010), Egyptian: an introduction to the writing and language of the Middle Kingdom, 2 vols. Egyptologische Uitgaven 24, page 97, note 3
Gardiner, Alan Henderson (1957), Egyptian grammar being an introduction to the study of hieroglyphs, 3rd, revised edition, page 490
Fischer, Henry George (1968), Dendera in the Third Millennium B.C. down to the Theban Domination of Upper Egypt., p. 134, fig. 24, Evolution of the sign's shape from the OK to the FIP.