While the exterior skin of the Guaranty expresses a new form for the steel skyscraper, its plan indicates those hard realities of function necessary to construct such a building and to sell it. The building is essentially a U-shaped plan stacked upon a rectangular solid. The interstitial spaces between wings of the "U" create opportunities to introduce skylights to the lobby below, and to cover the ceilings with stained glass. The plan contained a single vertical circulation core with four elevators, a mail slot, and staircase. No fire-stair was provided or necessary. The internal portion of the "U" faces south so as to collect light for the interior recesses of the building- light being a necessary commodity to attract good tenants. Sullivan spared nothing to accomplish this end for: "In order to increase the amount of light to the interior, the stairwell and the light slit facing the inner courtyard were lined with white glazed terra-cotta that was more costly than normal tiles."