What are the 7 Types of Lines in Technical Drawing?

Technical drawings use different types of lines to represent visible edges, hidden features, dimensions, centerlines, and cutting planes. According to standards such as ISO 128 and ASME Y14.2, each line type has a specific meaning.

The seven most common types of lines are:

Line Type	Appearance	Purpose
Visible (Object) Line	Thick continuous	Shows visible edges and outlines
Hidden Line	Thin dashed	Shows hidden edges and features
Center Line	Thin long dash–short dash	Indicates centers of circles, arcs, and symmetry
Dimension Line	Thin continuous with arrowheads	Shows the size of features
Extension Line	Thin continuous	Extends from the object to dimension lines
Section (Hatching) Line	Thin, evenly spaced diagonal lines	Indicates surfaces cut by a section plane
Cutting Plane Line	Thick chain line with arrows	Shows where an object is imagined to be cut for a sectional view

7 Types of Lines in Technical Drawing?

1. Visible (Object) Line

Appearance

Thick continuous line

Purpose

Represents all visible edges and boundaries of an object.

Uses

Object outlines
Visible corners
Visible surfaces

Example: The outer edges of a mechanical component.

2. Hidden Line

Appearance

Thin dashed line

Purpose

Represents edges or features that cannot be seen in the current view.

Uses

Holes behind surfaces
Internal slots
Hidden grooves

Example: A hole passing through a block that is not visible from the front.

3. Center Line

Appearance

Thin line with alternating long and short dashes

Purpose

Indicates the center of circles, arcs, cylinders, and symmetrical parts.

Uses

Hole centers
Shaft axes
Symmetrical components

Example: The centerline of a drilled hole.

4. Dimension Line

Appearance

Thin continuous line with arrowheads at both ends

Purpose

Shows the distance or size of a feature.

Uses

Length
Width
Diameter
Radius
Angle measurements

Example: A dimension line labeled 50 mm between two arrowheads.

5. Extension Line

Appearance

Thin continuous line

Purpose

Extends from the object to the dimension line to indicate the feature being measured.

Uses

Dimensioning
Measurement references

Example: Lines extending from the edges of a rectangle to a dimension line.

6. Section (Hatching) Line

Appearance

Thin, evenly spaced diagonal lines (usually at 45°)

Purpose

Represents surfaces that have been cut in a sectional view.

Uses

Sectional drawings
Internal features
Material identification (special hatch patterns may indicate different materials)

Example: A cross-section of a bearing or engine block.

7. Cutting Plane Line

Appearance

Thick chain line with arrows at the ends

Purpose

Shows the location where an imaginary cut is made to create a sectional view.

Uses

Section views
Internal component visualization

Example: A cutting plane labeled A–A through a machine part.

Additional Line Types (Commonly Used)

Besides the seven basic lines, technical drawings often include:

Line Type	Purpose
Leader Line	Connects notes or labels to features
Phantom Line	Indicates alternate positions or adjacent parts
Break Line	Shows that a portion of an object has been removed for clarity
Construction Line	Used as a guide during drawing; usually not part of the final drawing

Importance of Line Types

Using the correct line type helps:

Improve drawing clarity.
Avoid misinterpretation.
Communicate design intent accurately.
Ensure compliance with engineering drawing standards.
Facilitate manufacturing and inspection.

Summary

Line Type	Thickness	Function
Visible (Object) Line	Thick	Shows visible edges and outlines
Hidden Line	Thin	Shows hidden features
Center Line	Thin	Indicates centers and axes
Dimension Line	Thin	Shows measurements
Extension Line	Thin	Extends from the object to dimensions
Section (Hatching) Line	Thin	Represents cut surfaces in section views
Cutting Plane Line	Thick	Indicates the location of an imaginary cut

These seven line types form the foundation of technical drawing and are universally used in engineering, architecture, and manufacturing to communicate designs clearly and accurately.