Chapter 20-21(p. 22): When introducing Thor, the girdle is listed as one of his three special items, along with his hammer Mjöllnir and his iron gloves. Its function is explicitly described.
He has another possession that is very valuable, a girdle of might, and when he buckles it on his As-strength is doubled.
Chapter 3-4(p. 72): In a list of kennings (poetic names) for Thor, he is identified as the "ruler and owner of Miollnir and the girdle of might."
How shall Thor be referred to? By calling him son of Odin and lord... ruler and owner of Miollnir and the girdle of might...
Chapter 17-18(p. 81): The introduction to the story of Thor's journey to the court of the giant Geirrod explicitly states that Thor was tricked into going without his key equipment.
On that occasion he did not have the hammer Miollnir or the girdle of might or the iron gauntlets, and that was Loki’s doing.
Chapter 18(p. 82): Because he is without his own gear, Thor receives a replacement girdle from the giantess Gríðr, which he uses immediately.
She lent him a girdle of might and some iron gauntlets of hers, and her staff, called Grid’s pole. Then Thor approached the river called Vimur, greatest of all rivers. Then he buckled on the girdle of might and pressed down on Grid’s pole...
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