Hello
Every spirit level I have owned (you know, the standard Stanley thigs a yard long, with a vial filled with green liqwuid) has got two marks at each end of the vial - two thin lines close together at one end, and the same at the other.
Clearly when the bubble is centred the level is flat, but what's the graduation of these two lines at eaither end? If the bubble touches the inner line and then moves to touch the outer line, is it standard to be able to say that the inner line is so many degrees away from horizontal, and the second line is thus so many extra degrees?
DDS