Buying a new townhouse and one of the floors needs fixing because it’s not level.
Which makes me think , when I buy a level at the hardware store, who/what has decided that it measures the level correctly?
Not a big deal, just curious.
Gravity.
Plumb is straight up and down, and is found with a plumb-bob. A plumb-bob is a string with a weight on it.
Level is perpendicular to plumb, it is found by marking plumb and creating a perpendicular line by means of a 3-4-5 triangle or bisecting that line with a compass.
Levels are manufactured to float a bubble within a pair of lines to simplify this process.
Probably with some application of a plumb line (a length of string, twine, rope, etc. with a weight at one end) and a reference surface.
I recall seeing pictures of a medieval level that was just a triangle with a weighted line suspended from the apex of the triangle down into the interior.
If you think about it, the earth surface is not flat. So what are you making anything level, perpendicular or square to? Just food for thought.
Gravity.
Plumb is effectively a line pointing directly to the center of the earth. Level is perpindicular to plumb. Unless something is miles and miles long, the curvature of the earth is such a small variation that it can’t possibly be accounted for, nor will it make any appreciable difference.
Like the others said, level is perpendicular to gravity.
The levels you buy at the store attempt to be true to this non-arbitrary standard. Levels that do not meet the standard will measure differently if you turn them around 180 degrees.
The difference in the direction of the force of gravity over the area of something building-sized is so small as to be insignificant.
FWIW you can easily check a new level for accuracy.
Place it on a flat surface (Formica counter top is excellent for this)
Note where the bubble sits in relation to the lines. Rotate the level 180 horizontally and observe the bubble. Same place? Your level is accurate.
If worse comes to worse subbing lime jello in the vials will fix any out of level conditions you are likely to encounter.
You can also find true level between two points using clear plastic tubing. Run the tubing to the points and hold the ends vertical at those points. Fill the tube with water and the points where the water settles at opposite ends will be level with eachother.
Explain, please. Assume point A is nearest the ceiling and point B is in my right hand. Where are they after the rotation?
If you want to look at it like that, then level can be considered a tangent to the reference point on the earth.
Neither is level. Level is spherical.
Your language is just loose enough to invite confusion. I know you know what you meant, but I can read it a couple of different ways. I will start from scratch trying to use meticulously descriptive language.
Call the ends of the level A & B. if the level is resting on a true level table top then it will read the same when A is north or east or south or west. If the level is correctly calibrated, then this reading will be with the bubble centered. If the level has an error, then the same non zero reading will be observed for all orientations.
In practice you don’t typically have a known level table to use check your level. So instead you use any flat surface that is approximately level. You first take a reading with end A of the level to the south. Say your level indicates that the south end is a quarter bubble high. Now spin the level so that end B is to the south. If the level still indicates that the south end. (which is now B instead of A) is still 1/4 bubble high, then you know the level is good, and the reading is due to the surface being out of truth. If the level should give different readings, then you know the level is not reading correctly, and the surface is halfway between the two readings.
Most levels also have cross vials for checking plumb. You can use any approximately vertical surface the same way… Door jambs are often handy for this. In this case you check once with end A at the top, and then with B at the top.
Stand in front of the level put one end in each hand.
Take the end of the level that is in your right hand and put it in your left hand. Place the level back in the same place on the counter where it was before.
You have now rotated it 180 degrees horizontally.
But does gravity have anything to do with it? I thought somebody would have mentioned it by now.
In case the question is “How do you know a level is actually reading level properly?”, the answer is you don’t unless you check it with a plumb bob or an already established level surface. There are probably only simple QC procedures for most consumer levels, but they shouldn’t be difficult to manufacture with reasonable accuracy.
The simple way to test a level is to find a flat surface that reads level, and rotate the level around on the surface. It should read level no matter how you turn it.
There is at least 1 very long bridge that was built to take into account the Earth’s curvature.
(note, this article does not mention that fact)
I’m sure that large bridge-building is highly relevant to someone searching for a four-foot box level. 
Cool fact*: If you have a frictionless flat table, level at the center, and place an object on it at any point except the center, the object will oscillate with a period of about 84.4 minutes, the same period as an orbit that just skims the Earth’s surface.
- My “cool fact” identification system may be miscalibrated.
I toured a car plant and we were told that they had to take curvature into account.