Which tool is used to check or establish a true vertical line (plumb)?

Reference directions in construction: LEVEL equals HORIZONTAL (perpendicular to gravity, parallel to ground when ground is flat); PLUMB equals VERTICAL (parallel to gravity, perpendicular to a true horizontal); SQUARE equals at 90 degrees to another reference (often a corner or edge). Tools for each: SPIRIT LEVEL (BUBBLE LEVEL) is sealed glass vial partially filled with liquid (originally alcohol/spirit, hence the name) with a bubble; level when bubble is centered between marks; LEVELS have horizontal and vertical (plumb) vials; many also have a 45 degree vial; sizes from 9 inch torpedo to 8 foot levels; longer lengths give greater accuracy over distance. PLUMB BOB is pointed weight suspended from a string; gravity holds string vertical; for very accurate vertical reference over long distances (multiple stories); used by surveyors, masons. LASER LEVEL projects a laser beam or line; self-leveling (with pendulum mechanism) projects truly level/plumb lines; very accurate; modern construction standard; types include rotary (rotating point making horizontal plane), line lasers (projecting visible lines), cross-line (horizontal AND vertical at once). WATER LEVEL is clear hose filled with water; water surface in two ends will be at the same height (basic physics); used for establishing level over distances or around obstacles. SQUARENESS checks: TRY SQUARE has fixed 90 degree angle for small work; COMBINATION SQUARE is adjustable, multi-purpose (90 and 45 degrees, depth measurement); FRAMING SQUARE is large L-shape for layouts; tongue and blade; SPEED SQUARE is triangular layout for framing; SQUARENESS OF A RECTANGLE measured by both diagonals; if they're equal, the rectangle is square (geometric proof). For building a wall, deck, or any structure: mark and set corners by location (measurements); check PLUMB on uprights with level; check LEVEL on horizontal members with level; check SQUARE of overall structure with diagonal measurements or with 3-4-5 method (a triangle with sides 3, 4, and 5 units has a 90 degree angle between the 3 and 4 sides; mark 3 ft on one wall, 4 ft on adjacent wall; if distance between marks is 5 ft, it's square; multiples like 6-8-10 work for larger). LEVEL ACCURACY: check level on a known surface (set on table, mark position, flip 180 degrees; should read the same; if not, level is out of calibration). Modern quality levels are accurate; cheap ones often aren't. Digital levels show degrees precisely with a number. Check periodically; bumps and drops can affect accuracy. PLUMB BOB USE: suspend from high point; allow to settle (oscillation can take time); string is plumb when motionless; mark plumb point at base; for high accuracy, allow plenty of settle time; wind affects outdoor use. ESTABLISHING LEVEL: from a known reference point (existing foundation, established benchmark); use water level or laser for long distances; check periodically as work progresses. SPECIAL situations: when a floor slopes intentionally (drainage), level reference may not be horizontal but rather should match desired slope; when measuring from non-horizontal datum, use that reference; grounds out of level need shimming/leveling first.