Plumb means perfectly vertical — pointing straight at the center of the earth, the way a weight hangs on a string. Level means perfectly horizontal, at 90° to plumb. Walls, posts, door jambs and the plumb cuts on rafters and stringers must be plumb; floors, plates, headers, countertops and rafter seat cuts must be level. The third rule of framing is square: corners at exactly 90°, usually checked with the 3-4-5 method or by measuring both diagonals of a rectangle until they match.
The classic tool is a spirit level: the bubble centered in the horizontal vial means level, in the end vials means plumb. A 2 ft level works for short runs, but a 4 ft or 6 ft level bridges bumps in the lumber and gives a truer reading. A plumb bob (a pointed weight on a string) is still the most accurate way to transfer a point straight down, and rotary and line lasers now do the same job across an entire room.
“Out of plumb” is described as a distance over a height — for example 1/4” in 8 ft. Many carpenters aim to keep walls within about 1/8” over 8–10 ft so trim and cabinets fit without shims. Small errors compound: a wall 1/2” out of plumb at the top shows up as a visible gap along every upper cabinet.