"This was the fourth threshold we stepped through in these mountains. The map says nothing about it, so they must be new - I worry who would poke holes into the Here in six valleys."An Area threshold is where one can step over into the Area. Natural thresholds are, as many things about the Area, seemingly random - any crossing could lead into the Area, a riverbank, a door, a line between shadow and light.
— Szagrat, traveller
Artificial or drawn thresholds are easy to make, but require precision, and are best done by signmakers. Each of the four corners, then the opening, of the threshold-to-be is marked with the First Sign "Create Threshold". Because this last, fifth sign must therefore always be made into the air, and First Signs are complicated, it's hard to successfully create a threshold artificially.
"That was your tenth, no, eleventh, attempt at it. I suppose we can always wait for a Nightly Traveller to happen by when you're having such trouble with that damn threshold... I'm just joking!"
— Jagir, to Vramde
Because Genius Loci is also built as a roleplaying world, sometimes I think in terms of difficulties for dice rolls. But imagining how it actually looks is exciting; making a threshold should be mysterious and strange. Most travellers know how to make one, even if not all are good at it.