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Create a 2D spline
You can create a spline by specifying either a series of interpolation points through which the curve passes, or by defining a control polygon.
A spline created with interpolation points will pass exactly through the interpolation points used to define it.
Spline curves composed of a series of interpolation points can be modified. For more information, see Change a point on a spline.
When you create a spline using a control polygon, the shape of the spline is influenced by the control polygon formed by the control points. The spline will pass through the start and end points, and is influenced by the control points.
 If you attempt to simulate analytic geometry with splines (for example, by making straight lines or circles using splines), your part may later appear corrupt. To avoid this problem, use the Line and Circle commands for any geometry that you will want to reference later as analytical elements (faces, spheres, and so on).
To create a spline using interpolation points,
1. Click Modeling and then, in the Draw group, click the arrow next to Spline).
2. Click Interpolation under the Spline section. The Interpolation dialog box opens.
3. Click the workspace to set an interpolation point.
4. If you want to specify a tangent condition for the first point,
Enter an angle in the data entry field next to Tangent.
Click Tangent and enter an angle in the user input line, or specify a point for the angle.
5. If you want to change the tangent condition,
To clear the tangent, click off the Tangent indicator.
To change the tangent, use the methods described in step 6.
6. If you want to create a closed spline, Click Closed.
7. Click more points to define the shape of your spline. You can enter a tangent for any of them.
8. If you have specified a point incorrectly, you can step back through the points by clicking Back.
9. If you want to join your spline smoothly to other open elements of geometry, click Append to and click the end of the other elements. You can use both ends of the spline.
10. Click OK to complete the operation.
To create a spline using a control polygon,
1. Click Modeling and then, in the Draw group, click the arrow next to Spline).
2. Click Control under the Spline section. The Control dialog box opens.
3. If you want to create a closed spline, click Closed in the Control dialog box.
4. Click a series of points to define the shape of your spline in the workplane.
5. If you have specified a point incorrectly, you can click Back in the control dialog to step back through the points.
6. Click OK to complete the operation.