The Alphabet of Lines

Read the pattern before deciding what the line means

Every line on an engineering drawing has a job. Some describe the part; others tell you how to measure it, where to look, or how a view was made. Treating every line as a physical edge gives you the wrong part.

Use three clues together: pattern (solid or dashed), weight (thick or thin), and context (arrows, letters, numbers, and nearby features). A thin solid line alone could be a dimension, an extension, a leader, or section hatching.

Visible and hidden lines describe edges; center lines locate axes

Visible / object

A relatively thick, continuous line shows an edge or outline you can see from this viewing direction.

Hidden

A thin series of short, evenly spaced dashes shows an edge concealed by material. A hole may appear as a visible circle in one view and two hidden lines in another.

Center

A thin long-short pattern locates a center, axis, or symmetry reference. The center line through a cylinder is not a seam in the cylinder.

Phantom

A thin long-short-short pattern can show an alternate position, repeated detail, or a nearby part used as a reference. An open position of a lever does not mean there are two levers.

Read all views together. A line's visibility depends on where you are looking from. The same edge can be solid in one view and dashed in another.

Arrows, notes, and hatching tell you what to do with a view

Dimension

A thin line with arrowheads identifies the extent of a measurement. Read the associated value; do not measure the picture on your screen.

Extension

A thin line carries a feature's location out to a dimension line. A small gap usually separates it from the part outline.

Leader

A thin line connects a note to the feature it describes, usually with an arrow at an edge or a dot on a surface. Follow it to its endpoint.

Cutting plane

A prominent line marks an imaginary cut used to expose the interior. Read its arrows for the viewing direction and its letters to find the matching section, such as A–A. The line pattern can vary; arrows and identification matter.

Section / hatch

Thin parallel strokes inside a section view identify material crossed by the imaginary cut. They are not grooves machined into the part.

Break

A short freehand break or a long thin line with zigzags marks an omitted portion of a view. The actual part is not necessarily broken; a long uniform member can be shortened on paper while its dimension gives the full length.

For units and decimals, use Measurement Language. For allowable variation, use Tolerances and Fits. Creating the dimension callouts comes next in Drawing Dimensioning.

When lines coincide, do not invent a second feature

Where a visible edge and a hidden edge project to exactly the same place, read the visible line. Where a hidden edge and a center line coincide, read the hidden line; the center line may still be identifiable where it extends beyond the edge.

A crossing is different from an overlap. Two lines crossing on the sheet do not prove that two physical edges meet in space. Check another view before deciding.

Practice identifying lines