Join Polyline allows you to select a combination of polylines, lines and arcs and convert them into a single polyline. If a selected element is not a polyline, you are prompted on whether you wish to convert it to one. You can continue to select additional lines and arcs and join them to a single polyline entity by either using the shift key, or using your cursor to window all objects. This command is also handy when used in combination with the Export Profile to Profile command for preparing a series of lines and arcs to convert to a 3D profile.