Union symbol in an inscription at the temple of Amun-Ra in Karnak
The Union symbol (hieroglyph) is Gardiner sign listed no. F36, part of the series for parts of mammals. As a "union symbol", (a right and left half), it contains a vertical invisible 'centerline'. It allows for the positioning of two important hieroglyphs to be attached to it, right and left, as the uniting of two halves; specifically this is referencing Upper Egypt (by the King of the South), represented by the sedge hieroglyph (M23)
and Lower Egypt (the King of the North), represented by the papyrus clump hieroglyph (M16)
Budge, 1991. A Hieroglyphic Dictionary to the Book of the Dead,E.A.Wallace Budge, Dover edition, 1991; Original: c 1911 as: "A Hieroglyphic Vocabulary to the Theban Recension of the Book of the Dead with an Index to All the English Equivalents of the Egyptian Words", (Kegan Paul, etc. Ltd, London, publisher). Dover: (softcover, ISBN0-486-26724-5)
Schulz, Seidel. Egypt: The World of the Pharaohs, Editors, Regine Schulz, Matthias Seidel, Könemann Verlagsgesellschaft mbH, Cologne, English translation version. (hardcover, ISBN3-89508-913-3)