Extension snap

Use the Extension snap to snap to a temporary extension line of an arc or line. To make the extension line visible, pause the cursor over the endpoints of the entity.  

To set the Extension snap

Do one of the following to choose Extension ():    

    On the ribbon, choose Draw > Extension (in Entity Snaps).

    On the menu, choose Tools > Entity Snaps > Extension.

    On the Entity Snaps toolbar, click the Extension tool.

    Type extension and then press Enter.