How do I determine/control the number of spans in a curve?

In viewing the latest excellent tutorials on “Better Living Through CNC” on YouTube, the topic of curve degree and curve spans has come up. The Alias Golden Rules was mentioned as a good reference for this and its rule #3 discusses curve span. I can’t figure out how, in Rhino, how to determine and control how many spans a curve has. Can someone enlighten me?


A single-span curve has degree+1 control points. Any more than that, the curve is multi-span. So a degree 3 single-span curve will have 4 control points and a degree 5 single-span curve 6 control points.

Thanks. See the attached. Same curve drawn 2 ways. Have I interpreted these correctly?

Curves Spans.3dm (67.7 KB)

The first curve looks like two degree=3 curves that are joined.

See if this helps:

The WHAT command on the top curve returns this:

ID: dd79e855-f64e-4b92-b46b-91bba3d9adec (8033)
Object name: (not named)
Layer name: Default
Render Material:
source = from layer
index = -1
Active visual analysis modes:
Curvature graph analysis

Valid curve.
Open NURBS curve
start = (-1’-8 1/4",8 1/8",0")
end = (2 11/16",1’-2 1/4",0")
degree = 5
control points: non-rational, count=9
knots: non-uniform, domain = 0" to 3’-0 1/16"
clamped at start and end

How do I know how many spans there are from this information?

it kind of doesn’t matter how many spans it has if it’s above single span (degree + 1 control point)

If it’s not single span, then it’s a multi span curve and the single span rules kind of go out the window.

I guess I’m having trouble determining when I’ve gone from a single span curve with a lot of CV’s to a multi-span curve.

Did you read the page I linked about NURBS cuves?

If you are using degree 3 curves, more than 4 CV’s it’s multi-span…

Don’t forget the difference between two or more joined curves (a polycurve) and a single NURBS curve which may be single or multi-span…

1 Like

Greek to me but thanks anyway.

Sorry John, I didn’t mean to be flip earlier. The page you sent me to was way over my head I’m afraid.

Nothing to be concerned about.
My push back point is this is the level of technical detail you need to understand the specific questions you’re asking.

Here’s a link to the same information in a more friendly accessible format:
rhino_basics.pdf (2.5 MB)

Thanks John. Much more understandable.