A frontier, or borderline, is the space between two worlds, constituting a kind of double plot or action that the poet W. B. Yeats discovered to be the archetypal formula for producing "the emotion of multitude" or the sense of universality. In his essay "The Emotion of Multitude" Yeats explains and illustrates how in poetry and in art, the alignment of two actions without interconnection performs a kind of magical change in the interacting components.