What is Necessary And Proper?

Legal Definition
The Constitution of the United States, art. 1, s. 8, vests in congress the power " to make all laws, which shall be necessary and proper, for carrying into execution the foregoing powers, and all other powers vested by this constitution in the government of the United States, in any department or officer thereof."

2. This power bas ever been viewed with perhaps unfounded jealousy and distrust. is a power expressly given, which, without this clause, would, be im lied. The plain import of the clause is, that congress shall have all incidental and instrumental powers, necessary and proper to carry into execution all the express powers. It neither enlarges any power, specifically granted, nor is it a grant of any new power to congress. It is merely a declaration for the removal of all uncertainty, that the means of carrying into execution those already granted, are included in the grant.

3. Some controversy has taken place as to what is to be considered " necessary; "it has been contended that by this must be understood what is indispensable; but it is obvious the term necessary means no more than useful, needful, requisite, incidental, or conducive to. It is in this sense the word appears to have been used, when connected with the word " proper." 4 Wheat. 418-420; 3 Story, Cons-t. 1231 to 1253.
-- Bouviers Law Dictionary