In article .com
wrote:
I contributed the U - Tube to the DIY Laser Level thread as this is the
only rudimentary reference that I can think of for getting something
level.

In the vertical axis we have the "Plumb Line". This is what I call a
reference for getting something to stand or hang vertically.

I have asked other people what they consider is a horizontal refrence
instrument and they all say the "Spirit Level". This I disagree with
as it can be adjusted or altered.

You just turn it the other way and check if it's still level.