In Norse myth, existence is divided into realms, collectively known as the Nine Worlds: Asgard (realm of the Aesir gods), Vanaheim (realm of the Vanir gods), Alfheim (realm of the light elves), Midgard (realm of humans), Jotunheim (realm of the giants), Nidavellir (realm of the dwarves), Niflheim (realm of ice), Muspellheim (realm of the fire giants), and Helheim (realm of the dishonorable dead). Yggdrasil (the World Tree) connects the worlds and, in The Sword of Summer, has roots in Boston. Magnus and his friends journey to several of the Nine Worlds throughout the book.

As a mortal, Magnus is from Midgard, and a good portion of the book takes place in this world. When Magnus and Blitz travel to Nidavellir, Magnus notes the realm resembles Southie (South Boston). Blitz explains Midgard takes its appearance and popular culture partly from Midgard, as is evidenced by a Taylor Swift song playing in one of the pubs. When the group meets Thor, they travel to Jotunheim, realm of most giants. The exceptions are ice giants, who live in Niflheim, and fire giants (including Surt), who reside in Muspellheim. Giants take many forms and sizes, and those of the ice and fire realms are part giant, part demon.