The role of the Vertex Unit is to be placed in the model’s vertices. Compare this variant with the regular version.
Just as in the regular vertex unit, in the sunken vertex unit the flap can be placed on either side of the unit so that it is hidden from view and so that
the big crease runs along an edge and not through the middle of the unit’s face. The module shown above (from both sides) will look well when seen
from the side the small pyramid’s tip lies, but displays an ugly crease on the opposite side. If the flap was folded to the other side, the big crease
would move to the other side as well.