You can join a line, arc, or other polyline to an open polyline if their ends touch. If a line crosses the end of a polyline in a T shape, the objects can’t be joined. If two lines meet a polyline in a Y shape, AutoCAD selects one of the lines and joins it to the polyline. Joining also causes an implicit decurve, with AutoCAD discarding the spline information of the original polyline and any polylines being joined to it. Once the joining is completed, you can fit a new spline to the resulting polyline.