- This is the electrical version of Beta vs VHS. This plug can fit both our parallel style plugs and Tandem plugs. The parallel style plugs are the type we use today. The un-grounded type like you would find on a lamp can still operate in an outlet like this. The Tandem style plug has two horizontal flat blades. They fell out of favor in the early part of this century. The tandem parallel combo outlet was still sold in store up to the 1960. This type of outlet is almost always attached to Knob and Tube wiring.