A couple of the commonly misunderstood parts of the Constitution are the Necessary and Proper clause, and the Supremacy clause. I thought I would publish what Alexander Hamilton wrote on the subjects from the Federalist #33. Remember that these papers where written during the debate over ratification, and widely circulated. They informed the ratifiers to the understanding of the document they where about to sign, and as such they are similar to writing in the borders of a written contract, they are legally relevant, and enforcable.
By way of a general explaination or summation of Hamiltons sentiments I offer this short summary. The Necessary and proper clause is simply declaring that the way the legislature would carry out their enumerated powers would be to pass laws that are incidental to their powers, and the Supremacy clause simply stated the obvious that in powers pursuant to the Constitution the laws of the union would be supreme. In areas not pursuant to the Constitution however the laws of the states would still be supreme… DUHHH.
Without further hesitation, here is Hamiltons paper on the 2 clauses:Details