Line Characteristics and Visible Elements

Technical drawings use different line styles to represent various aspects of an object. Lines can be continuous (solid), dashed (separated by spaces), and vary in line weight (thick or thin) to communicate specific information.

Visible lines are thick, continuous lines that represent the edges and outlines you can actually see on an object. Think of these as the lines you would trace if you were holding the object in front of you.

Hidden lines show edges that exist but can't be seen from your viewing angle—they're behind the object or inside it. These appear as thin, evenly spaced short dashes on your drawing.

Section lines (thin diagonal lines) appear only when showing a "cut" surface in sectional views. They help identify different parts or materials by changing direction of the diagonal pattern.

Pro Tip: When using center lines to mark the middle of circles or holes, make sure the long dash extends beyond the feature, making it easier to locate the exact center point.

Specialized Line Types for Measurements and References

Symmetry lines indicate when both sides of a part mirror each other. They look similar to center lines but have two short thick parallel lines placed at both ends. These are useful in partial views where showing the complete object isn't necessary.

Dimension lines and extension lines work together to communicate measurements. Dimension lines (thin with arrowheads) show the actual measurement, while extension lines extend from the object to show exactly what's being measured.

Leader lines are thin slanted lines with arrowheads that point to specific features needing explanation. They connect notes, symbols, or part numbers to the exact location they reference.

Cutting-plane lines and viewing-plane lines help readers understand different perspectives. Cutting-plane lines (thick with oversized arrows) show where a part is "cut" for sectional views, while viewing-plane lines indicate the direction to look at for alternate views.

Remember: The thickness of lines isn't random—it creates a visual hierarchy that helps your brain quickly interpret what's most important on a technical drawing.

Specialty Lines for Special Situations

Phantom lines (thin lines with one long line followed by two short dashes) show alternative positions of moving parts or extra material before machining. They help you visualize movement or material changes without cluttering the drawing.

Break lines indicate when parts of an object have been removed from the drawing. They come in three types:

• Short break lines: thick, wavy freehand lines for small breaks
• Long break lines: thin lines with zigzags for lengthy uniform parts
• Curved break lines: used on cylindrical objects to suggest material texture

Stitch lines represent sewing processes on parts or assemblies. These thin, short dashed lines (3mm) of equal size and spacing show where components will be stitched together.

Chain lines are thick dashed lines with a repeating pattern of a long line broken by a short line. They indicate special treatments for specific surfaces or projected tolerance zones.

Quick Check: If you're ever confused about which line to use, ask yourself what information you need to communicate—is it visible, hidden, a measurement, or a special instruction?