Well Don I am not sure why you have such an attitude about all this because it is an open discussion not court , but maybe you are just like that. So if I am understanding you correctly , about every house out there is in violation of " the law" ,since all of them are using appliances the have 16 ga and some 18 ga wire ?

Actually, I do not have an attitude about this and no having an extension cord is not a code violation.

Let me try it this way, It is a violation of the NEC to connect a #14 AWG (copper) conductor to a #12 AWG (copper) circuit and run the circuit at 20 amps.

If the house is wired with aluminium wiring, then #12 (AL) cannot be married to #10 (AL) without reducing the amperage to 15 amps.

This applies to the wire behind the wall to the receptacle, running from the circuit breaker to the receptacle.

Yes I agree with that as you have written it. The last comment did not clarify it was just the "branch Circuit" from the panel. Nmm