The Ancient Egyptian Mat hieroglyph, (originally for a stool), and later the Stool hieroglyph, Gardiner sign listed no. Q3 originally was the stool's rectangular mat; later it became the representation for the stool itself.
The Mat or Stool hieroglyph is used in the Ancient Egyptian language hieroglyphs for the alphabetic consonant letter p.
The Egyptian hieroglyph alphabetic letters
The following two tables show the Egyptian uniliteral signs. (24 letters, but multiple use hieroglyphs)
References
Stool-or-mat (hieroglyph) Wikipedia(Text) CC BY-SA