I see two kinds of 4' spirit levels at the Borg: one kind with just one straight horizontal vial (like the FatMax), and others with two curved vials (both curve away from each other). I note that when I hold one edge of the dual vial against a level surface, the vial closest to that edge shows level, while the vial above it is not level. I assume the knowledgeable user knows which edge to associate with "level."
What is this kind of level used for, and what are the advantages of this dual vial?
TIA, Chuck