Thresholds may be physical, marking a gradient between two places or states of being, or metaphorical, marking some Perception of change. In both instances, they may contain and be focused by Portals, but, while a portal always signals a threshold of some sort, a threshold does not require a focus. They may not even be meant to be liminal, or passable.
Physical thresholds normally form the spines of Borderlands, demarcating regions which borderlands join together; they announce the presence, or intrusion, of a Crosshatch; they constitute the perimeter of Polders; and, for those of peculiar Talents, they may comprise a Map of the Land.
Metaphorical thresholds are part of the sustaining warp and woof of a literature which addresses the "essence" of Landscape, and in which all transitions – anything which the author deems worthy of notice – are ipso facto significant. Thresholds tend to mark, therefore, jointures in plot and other points in the grammar of Story; a whiff of difference (threshold moments are often described in terms resembling synaesthesia) may augur Bondage or Wrongness or Thinning; thresholds of anticipation, often conveyed as a welling up of the pressure of story, may circumambiate the moment of Recognition in a full fantasy, the moment when characters and land pass the barrier into right knowing, Metamorphosis, proper remembrance or Healing.
The heart of all this is the sense that, in fantasy, where there is theoretically nothing that one cannot say (and therefore no excuse for saying something unmeant), thresholds are maps to the meaning of the text. [JC]