From Wiktionary, the free dictionary
Representing the sky with an architectural rather than mythological image, as a gigantic ceiling resting on four supports. The phonogrammatic value is derived from the glyph’s use in ḥrt ( “ sky ” ) .
(ḥrj )
Triliteral or biliteral phonogram for ḥrj or ḥr , as in ḥrj ( “ being above ” ) .
Logogram for pt ( “ sky ” ) .
Logogram for ḥrt ( “ sky ” ) in jn-ḥrt .
Determinative in words for the sky, as in nwt , ḥrt , pt .
Determinative in words for gates, by confusion with other glyphs, as in rwtj ( “ double gate ” ) , hꜣyt ( “ portal ” ) .
Gardiner, Alan (1957 ) Egyptian Grammar: Being an Introduction to the Study of Hieroglyphs , third edition, Oxford: Griffith Institute, →ISBN , page 485
Betrò, Maria Carmela (1995 ) Geroglifici: 580 Segni per Capire l'Antico Egitto , Milan: Arnoldo Mondadori Editore S.p.A., →ISBN