Lineweight is the visual thickness of lines. The use of lineweight is critically important to maintaining good legibility and professional appearance in drawings.
The following pair of drawings of the Robie House in Chicago illustrate conventional uses of lineweights:
Discerning objects cut by a plane. Relatively heavy lineweights (e. g. in excess of 0.5 mm) are usually used to designate objects cut by a plane. For example, in a plan drawing, a lineweight of 0.5 mm might designate walls and columns; in a section drawing, the same lineweight might designate floors, walls, and the ground.
Discerning changes in observable planes. Relatively medium lineweights (e. g. 0.3 mm or so) are usually used to designate objects observed against a background. For example, in a plan drawing, a lineweight of 0.3 mm might designate a countertop, an item of furniture, stairs, a balcony edge, and so on; in a section or elevation drawing, the same lineweight might designate the outline or edge of a wall against a distant wall, or an opening in a wall. In an axonometric drawing (such as the one of the Robie House to the left), a medium or heavy line might outline solid objects against a background.
Discerning changes in material occurring on a single plane. Relatively thin lineweights (e. g. less than .1 mm) are usually used to designate changes in material occurring on a single plane. For example, in a plan drawing, a lineweight of less than 0.1 mm might designate a threshold at a door, or the spring point of a ramp; in a section or elevation drawing, the same lineweight might designate trim around a door or window.
Designating material or texture. The thinnest possible lineweight (0 mm) is often, though not exclusively, used for hatch patterns in plans, sections, and elevations. Note that 0 mm does not indicate “no line”; rather, it indicates the thinnest line producible by the selected output device.