Posted by LordOfBikes on Aug 16, 2019; 10:51pm
URL: https://forum.librecad.org/LibreCAD-equivalent-of-AutoCAD-JOIN-command-tp5717648p5717663.html

When you say curve, I assume you try to join splines or elliptical arcs.
This doesn't work because LibreCAD has no approximation method to convert splines or ellipses to a set of  circular arcs.
But polylines can only contain arcs and lines, so joining curves doesn't work.

The condition for the join tool is that all the entities to connect build a continuous contour where start and end points match exactly. The entities to connect can be lines, arcs or polylines.