If you select a 3D polyline, the following prompt is displayed:
The following prompts are displayed.
Enter an option [Close/Join/Edit vertex/Spline curve/Decurve/Reverse/Undo]: Enter an option or press Enter
If the polyline you select is closed, Open replaces the Close option in the prompt.
Creates the closing segment of the polyline, connecting the last segment with the first. The polyline is considered open unless you close it with Close.
Removes the closing segment of the polyline. The polyline is considered closed unless you open it with Open.
Joins an open curve to the 3D polyline. The curve can be on a different plane, but must be contiguous with the 3D polyline.
Moves the X marker to the next vertex. The marker does not wrap around from the end to the start of the polyline, even if the polyline is closed.
Moves the X marker to the previous vertex. The marker does not wrap around from the start to the end of the polyline, even if the polyline is closed.
Saves the location of the marked vertex while you move the X marker to any other vertex.
If one of the specified vertices is at an end of the polyline, the polyline is truncated. If both specified vertices are at endpoints of the polyline, or if just one vertex is specified and it is at an endpoint, you cannot use Break mode.
Saves the location of the marked vertex while you move the X marker to any other vertex.
To remove an arc segment that connects two straight segments of a polyline and then to extend the straight segments until they intersect, use the FILLET command with a fillet radius of 0.
Fits a 3D B-spline curve to its control points. The SPLFRAME system variable controls the accuracy and display of the control points for the 3D B-spline, whose curves can be approximated only by line segments. Negative values for spline segments are ignored.

Removes extra vertices inserted by a fit or spline curve and straightens all segments of the polyline. Retains tangent information assigned to the polyline vertices for use in subsequent fit curve requests. If you edit a spline-fit polyline with a command such as BREAK or TRIM, you cannot use the Decurve option.