Line weight is a channel of information running alongside the geometry. A reader scanning a busy plan gets the structure of the building from the dark lines alone, before reading a word. Every sample below is drawn at the weight and dash pattern the sheets in this set are actually drawn with.
Weights
Weights are stated in inches of ink on the paper, so they do not change when a drawing is drawn at a different scale. A heavy line is heavy on a plan at one eighth of an inch to the foot and on a detail at one and a half inches to the foot, because the weight belongs to the paper rather than to the building.
Heavy, 0.022 inchWhatever the section cut through: walls and columns on a plan, floors and roof on a section, and the border of the sheet itself.
Medium, 0.012 inchWhat is seen but not cut: fixtures, casework, stair treads, door leaves, equipment, and the outline of things below the cut.
Light, 0.006 inchAnnotation and reference: dimension lines, extension lines, leaders, hatch, and anything that describes the drawing rather than the building.
Hairline, 0.004 inchThe finest tone on the sheet, used for dense hatch and for a screened background belonging to another discipline's drawing.
Dash patterns
A dash pattern says something about the relationship between the line and the view, not about the thing itself. A beam is solid on a framing plan, where it is the subject, and dashed on an architectural plan, where it is above the ceiling and out of sight.
SolidSomething real and visible in this view. The default, and the one that needs no explanation.
DashedSomething above the cut or otherwise not visible from here: a beam over, a soffit, an upper cabinet, a roof overhang.
HiddenA shorter dash, used for something behind or below: piping under the slab, a footing below grade, an element concealed in a wall.
CentreA long dash and a short dash, repeating. It marks a reference rather than a thing, which is why grid lines use it and why nobody builds one.
The same lines in use
The three below are drawn by the sheet renderer rather than as samples, so they carry their annotation as well as their weight. A cut wall reads as a filled band, a grid line reads as a centre line with a bubble, and a pipe below the slab reads as a hidden line with its size and service.
Cut wallHeavy and filled, because the plan cut through it. The band is the real thickness.
Grid centre lineA reference line, drawn so it can never be mistaken for a piece of building.
Pipe below the slabHidden, because it is below the cut, and labelled with its size and its service.