A shen ring is a circle with a line at a tangent to it, which was represented in hieroglyphs as a stylised loop of a rope. The word shen itself means, in ancient Egyptian, encircle, while the shen ring represented eternal protection. In its elongated form the shen ring became the cartouche which enclosed and protected a royal name.
The Shen ring is most often seen carried by the falcon god Horus, but was also carried by the vulture goddess Nekhbet. It was used as early as the third dynasty where it can be seen in the reliefs from Djoser's Step Pyramid complex.
The symbol could be stretched to contain other objects, which were then understood as being eternally protected by the shen ring. When it contained the name of the pharaoh the symbol became the cartouche. The word shen itself means, in Egyptian, encircle.
The Shen ring is the 'shenu'.
Shen ring
Talisman of Osorkon C at the Louvre Museum, France
God Huh
Detail of pillar at the Temple of Kom Ombo
The Temple of Hatshepsut and its vast quantity of Shen rings.
Stela at the Louvre
Stela of Harpist and god Ra
Seshat carved on the back of the throne of the seated statue of Rameses II.