"alexy" wrote in message
...
I got ready to measure flatness of my bench, and realized that I don't
know exactly what is meant by the various measures I have seen.

I've seen statements about a table's "flatness", expressed in
thousandths of an inch or in thousandths of an inch per foot, e.g flat
to .002" or flat to .002"/ft. Are people using those interchangeably,
or does the first mean literally over the entire surface, deviation
from one point to another is no more than .002? And in the second
measure, does that mean that using a 1-ft straight-edge, any dip in
the middle is less than .001" and any rise in the middle will cause
one end of the straightedge to be high by .002" when the other edge is
held down?

It's a matter of relativity.

Flat is good enough for what's it's doing. If pieces don't rock on your
bench, either both are flat enough or both OTL.