By the way, action takes place on Yarth, a place somewhat similar to Oerth, the setting of Greyhawk, et al. It has fewer magical properties than Oerth but more than Earth. It is not impossible that additional works will be contracted for in months to come, action being set on Yarth or perhaps another alternate world, Aerth. On Earth, magic is virtually non-existent. On Uerth, dweomers are weak, chancy things. Yarth has a sprinkling of things magical, and Oerth is pure magic.

Gary Gygax, quoted at http://thezerolevel.com/cheapskate-reviews-internet-archive-gamebooks/