A method of presenting a digital work includes displaying a portion of the digital work on a display screen under a set of display conditions, and providing one or more invariant location reference identifiers corresponding to the portion of the digital work on the display screen. The invariant location reference identifiers are separate from the digital work, and each invariant location reference identifier is provided along with the corresponding portion of the digital work, regardless of the display conditions under which the portion of the digital work is displayed.