In the world of AutoCAD, not all design elements are straightforward lines or perfect circles. When your drawing needs to follow real-world complexities, such as curving roads, organic product shapes, or artistic flourishes, standard line tools may fall short. This is where Polylines and Splines truly shine.

Polylines and Splines are essential tools for creating flexible, flowing, and complex designs with precision and ease. These tools will boost your drawing efficiency, accuracy, and editing capabilities.

Why Polylines and Splines Matter

When working on technical drawings or creative designs, you’ll often encounter geometry that isn’t easily represented by simple straight lines. For example:

• Site boundaries that follow curved roads
• Product designs with smooth, ergonomic curves
• Mechanical parts with precision arc sequences
• Artistic or conceptual drawings with flowing, freeform shapes

To handle these cases, AutoCAD provides two specialized tools:

• Polylines (PLINE): A connected series of line and arc segments treated as a single object.
• Splines: Smooth, mathematically generated curves that pass through or near specified points.

Both tools allow you to:

• Create complex shapes more efficiently
• Edit entire shapes without breaking them into individual line segments
• Apply special properties like width, curvature, or elevation

Scenarios

Examples:

• An urban planner is designing a site plan that includes irregular property boundaries. The lines must follow a combination of straight lot lines and winding roads. Using individual lines would result in a disorganized and hard-to-edit mess. Polylines let the planner create a clean, continuous, and editable boundary.
• An industrial designer is modelling a vehicle body. Smooth, aerodynamic curves are essential for form and function. Lines and arcs are too rigid. By using Splines, the designer creates the flowing shape needed for wind resistance testing and manufacturing.

Both users rely on Polylines and Splines to balance flexibility, control, and accuracy. Both have qualities that are hard to achieve with basic CAD tools alone, such as Lines and Arcs.

Understanding Polylines (PLINE)

What is a Polyline?

A polyline is more than just a set of connected lines. It’s a single object that can include both straight and curved segments. You can:

• Modify the entire shape with a few clicks
• Change line width, elevation, and linetype
• Easily adjust the geometry using grips or editing commands

How to Create a Polyline

1. Command: Type PLINE or PL into the command line.
2. Specify the starting point.
3. Continue clicking to define each new point. For arcs, choose the Arc option (Right-Click and select Arc).
4. Close the shape by pressing C at any point or just finish with Enter.

Editing Polylines with PEDIT

The PEDIT command is your go-to tool for polyline editing:

• Join separate lines and arcs into one polyline
• Convert line segments to arcs (or vice versa)
• Add or remove vertices
• Adjust width at specific points or throughout the polyline

Understanding Splines

What is a Spline?

A Spline is a smooth, curved line generated using fit points or control vertices. It’s perfect for:

• Product design
• Freeform architecture
• Organic forms and artwork

While more flexible than polylines, Splines are also slightly more complex in behaviour and output.

How to Create a Spline

1. Command: Type SPLINE
2. Click to define fit points. AutoCAD automatically creates a smooth curve through (or near) these.
3. Press Enter to complete the spline.

Modifying Splines

• Use grips to adjust curve tension and direction interactively.
• Want to prepare a spline for CNC or laser cutting? Use SPLINEDIT to convert the spline to a polyline for compatibility with manufacturing workflows.

Polylines vs. Splines: Quick Comparison

Feature	Polyline	Spline
Type	Connected lines/arcs	Smooth, freeform curve
Editing	Easy with grips or PEDIT	Edit with grips or control vertices
Precision	Great for mechanical, architectural designs	Best for natural, organic forms
Manufacturing Output	Preferred format for CNC	Often needs conversion to polyline
Example Use	Lot boundaries, piping layouts	Car bodies, artistic patterns

Design a Shape Using Both Tools

Pro Tip: Joining Objects into a Polyline

Got a messy drawing with disconnected lines and arcs? Clean it up using:

• PEDIT → Select a line → Join
• Select all the segments you want to merge

This transforms your chaos into a tidy, single polyline, creating a single object. It is easier to modify, export, or plot a joined polyline and adding the correct lineweight, line colour and linetype to 1 entity/element/object, as it is working with multiple single lines.

Conclusion

Polylines and Splines are foundational tools that allow AutoCAD users to create, modify, and present complex geometry with control and clarity. Whether you’re an architect outlining property lines, a designer crafting product shapes, or a drafter cleaning up a drawing, these tools are your go-to for precision and efficiency.

With a bit of practice, you’ll wonder how you ever managed without them.

Additional Resource:

How to convert splines to polylines in AutoCAD

AutoCAD 2024 Help | To Join Polylines, Splines, Lines, and Arcs Into a Single Polyline | Autodesk

AutoCAD Quick Start Guide – Polylines in AutoCAD | Autodesk