A spline is a smooth curve that passes through or near a set of points that influence the shape of the curve.
Draw a Spline
Click Home tab > Draw panel > Spline.
(Optional) Enter m (Method). Then enter either f (Fit Points) or cv (Control Vertices).
Specify the first point of the spline.
Specify the next point of the spline. Continue specifying points as needed.
Press Enter to end or enter c (Close) to close the spline.
Draw a Spline Tangent to and Connecting Two Lines or Curves
Click Modify tab > Blend Curves.
(Optional) Enter con (Continuity) and enter a tangency option.
Select a line or curve near an endpoint.
Select another line or curve near an endpoint.
Convert a Spline-Fit Polyline to a Spline
Click Home tab > Draw panel > Spline.
Enter o (Object).
Select a spline-fit polyline and press Enter.
The selected Object changes from a polyline to a spline.