Splines are non-uniform curves, which are used to create irregular shapes. In AutoCAD, you can create splines by using two methods: Spline Fit and Spline CV. These methods are explained in this autocad 2018 tutorial.
A spline is a smooth curve that passes through or near a set of points that influence the shape of the curve.
By default, a spline is a series of blended curve segments of degree 3 (also called cubic) polynomials. These curves are technically called nonuniform rational B-splines (NURBS), but are referred to as splines for simplicity. Cubic splines are the most common, and mimic the splines that are created manually using flexible strips that are shaped by weights at data points.
SPLINE creates curves called nonuniform rational B-splines (NURBS), referred to as splines for simplicity.
Splines are defined either with fit points, or with control vertices. By default, fit points coincide with the spline, while control vertices define a control frame. Control frames provide a convenient method to shape the spline. Each method has its advantages.
To display or hide the control vertices and control frame, select or deselect the spline, or use CVSHOW and CVHIDE. However, for splines created with control vertices in AutoCAD LT, you can display the control frame only by selecting the spline.
The prompts differ, depending on whether you choose Fit or CV (control vertices) as the creation method (Method option).

• Understand Control Vertices (spline cv) and Fit Points (spline fit):
You can create or edit splines using either control vertices, or fit points. The spline on the left displays control vertices along a control polygon, and the spline on the right displays fit points.
Use the triangular grip on a selected spline to switch between displaying control vertices and displaying fit points. You can use the round and square grips to modify a selected spline.


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