What is a ʼClosedʼ curve vs. an ʼOpenʼ curve in Rhino?

Short Answer

In Rhino, a closed curve forms a complete loop with coincident start and end points, while an open curve has separate ends. The fastest professional way to check this is with Properties, which shows whether a selected curve is open or closed. A curve can still appear closed visually while failing technically.

What You Need to Know Before

Warning: Curves that look closed on screen may actually contain tiny gaps, overlapping ends, or duplicated segments. This often causes failures in planar surfaces, extrusions, hatching, and CNC or laser-cutting workflows.

How to Check if a Curve Is Open or Closed

Command: Properties
Shortcut: F3
Quick Steps:
1. Select the curve in the viewport, then open the Properties panel if it is not already visible.
2. In the object details, check whether Rhino reports the curve as Open or Closed.
3. If needed, run SelOpenCrv or SelClosedCrv to quickly find open or closed curves in the model.

Variables & Settings

Key Setting: In the Properties panel, Rhino reports the curve status directly as Open or Closed.

Why it Fails

Cause 1 (Geometry): The curve endpoints are not actually touching, or the curve contains a microscopic gap that prevents it from being closed.
Cause 2 (layers/Locks): The curve may be on a locked layer, preventing edits needed to close it or join related segments.
Cause 3 (Command/Logic): Multiple segments may be joined visually but still remain open because Join cannot close gaps beyond model tolerance.

Quick Fix & Best Practice

Quick Fix: Use Join on touching curve segments, then confirm the result in Properties. If endpoints are slightly apart, use CloseCrv only when the shape should logically be closed.
Manager’s Verdict: Always verify closed curves before creating surfaces, hatches, or fabrication geometry. In production workflows, never trust visual appearance alone—check curve status explicitly.