How to join two separate lines into one single curve in Rhino?

Short Answer

Yes — in Rhino 3D, the most common way to join two separate lines into one single curve is to use Join after making sure the line endpoints touch exactly. This creates one joined polycurve object for editing and selection. Limitation: Join does not turn angled segments into a smooth single-span curve.

What You Need to Know Before

Warning: If the two lines do not share the same endpoint location within Rhino’s file tolerance, Join will fail even if they look connected on screen. Also, if one object is locked or the lines overlap instead of meeting end-to-end, Rhino will not create one valid joined curve.

How to Join Two Separate Lines into One Single Curve

• Command: Join

• Shortcut: Join

• Quick Steps:

  1. Select both lines in the viewport, or run Join from the command line/menu.
  2. Press Enter to join the selected curves into one object.
  3. If Rhino does not join them, turn on End Osnap, move one endpoint so both ends touch exactly, then run Join again.

Variables & Settings

• Key Setting: Absolute tolerance

• Expert Setting: Rhino uses the document’s absolute tolerance to decide whether endpoints are close enough to join. If the gap is larger than that tolerance, Join will fail, so check File > Properties > Units before forcing geometry together.

Why it Fails

• Cause 1 (Geometry): The line endpoints are slightly apart, overlapping, or not touching end-to-end within file tolerance.

• Cause 2 (layers/Locks): One or both lines are on a locked layer or are individually locked, so Rhino cannot modify them into a joined object.

• Cause 3 (Command/Logic): Join creates a polycurve from connected segments; it does not rebuild two separate lines into one smooth NURBS curve automatically.

Quick Fix & Best Practice

• Quick Fix: Use Move with End Osnap to snap one endpoint exactly onto the other, then run Join.
• Manager’s Verdict: Use Join when you need one editable curve object from connected line segments. If manufacturing or surfacing requires true smooth curvature, rebuild or redraw the result instead of stopping at a joined polycurve.

FAQ

Can Join make two lines into one smooth curve?
No, it normally creates a polycurve, not a smooth single-span curve.

Why do my lines look connected but won’t join?
Their endpoints are usually outside the file tolerance.

How do I check whether Rhino made one object?
Select it and confirm Rhino reports one closed or open polycurve in Properties or the command line.

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