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. This is why I say the "U - Tube is
the reference for horizontals as there is no adjustment, you just wait
for the liquid to settle.

What did the Egyptians, Romans et al use for horizontals. They could
have constructed a form of U-Tube but not a moveable/flexible one.

OK Pundits, what do you rekon?

Chris