Joining polylines    

You can add an arc, line, or polyline entity to an existing open polyline, forming one continuous polyline entity. To join an entity to a polyline, that entity must already share an endpoint with an end vertex of the selected polyline.    

When you join an entity to a polyline, the width of the new polyline segment depends on the width of the original polyline and the type of entity you are joining to it:

    A line or an arc assumes the same width as the polyline segment for the end vertex to which it is joined.

    A polyline joined to a tapered polyline retains its own width values.

    A polyline joined to a uniform-width polyline assumes the width of the polyline to which it is joined.    

Use the Join command to join arcs and lines.
For more details, see Joining entities.

To join an arc, line, or polyline to an existing polyline

   1   Do one of the following to choose Edit Polyline ():

    On the ribbon, choose Edit > Edit Polyline (in Modify).

    On the menu, choose Modify > Edit Polyline.

    On the Modify toolbar, click the Edit Polyline tool.

    Type editpline and then press Enter.

   2   Select the polyline.

   3   In the prompt box, choose Join.

   4   Select the arc, line, or polyline to join.

    5    In the prompt box, choose another option, or choose Done to complete the command.