Generally, Americans treat the Ninth Amendment like the Bill of Rights’ unwanted stepchild. They mostly ignore it, and when they do talk about it, they often misunderstand it.
In reality, the Ninth Amendment serves a very simply but crucial purpose. It expands the limits on the federal government – at least in a manner of speaking. More accurately, it makes the implied limits on federal authority explicit.
The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.”
Technically speaking, the Ninth and Tenth Amendments serve as “rules of construction.” They tell us how to “construe” the powers delegated to the federal government. They add nothing at all to the Constitution, and they don’t take anything away. They merely tell readers how to interpret the document.
Together the Ninth and Tenth Amendments affirm what was already understood and implicit in the Constitution – the federal government may only exercise the powers that were expressly delegated to it.
Many of the states ratified the Constitution on the condition of amendments. They thought a bill of rights was necessary to secure essential liberties and to ensure the federal government operated in the limited way promised by supporters. The New York ratifying document spelled out the understanding of that state’s ratifiers and the conditions under which the state would enter the Union.
Every power, jurisdiction, and right, which is not by the said Constitution clearly delegated to the Congress of the United States, or the departments of the government thereof, remains to the people of the several states, or to their respective state governments, to whom they may have granted the same; and that those clauses in the said Constitution, which declare that Congress shall not have or exercise certain powers, do not imply that Congress is entitled to any powers not given by the said Constitution; but such clauses are to be construed either as exceptions to certain specified powers, or as inserted merely for greater caution.“ [Emphasis added]
Several other states included similar provisions along with other proposed amendments. They ultimately formed the foundation for what would become the Bill of Rights. The highlighted phrase above was the genesis of the Ninth Amendment.
In simplest terms, the Bill of Rights sprung from fears that the federal government would act outside of its delegated powers and trample the people’s rights and privileges. The Ninth Amendment was a response to concerns that listing certain rights in a bill of rights would imply the federal government had the power to violate other rights not listed.
James Madison initially submitted proposals for amendments to Congress on June 8, 1789. His proposed verbiage was altered and generally shortened during the legislative process, but looking at the original language helps clarify the intent of the amendments. Madison’s proposal for what became the Ninth Amendment makes clear the intent was to amplify and make explicit the limits of federal power – specifically to ensure the enumeration of certain rights was not taken to imply the federal government could violate rights that were not mentioned.
The exceptions here or elsewhere in the constitution, made in favor of particular rights, shall not be so construed as to diminish the importance of other rights retained by the people; or as to enlarge the powers delegated by the constitution; but rather as actual limitations of such powers, or as inserted merely for greater caution.”
Some supporters argued a bill of rights wasn’t even necessary because the very nature of the Constitution precluded the federal government from taking actions that would violate rights like freedom of the press and religion. Theophilus Parsons advanced this line of thinking during the Massachusetts ratifying convention.
Mr. Parsons demonstrated the impracticability of forming a bill, in a national constitution, for securing individual rights, and showed the inutility of the measure, from the ideas, that no power was given to Congress to infringe on any one of the natural rights of the people by this Constitution, and, should they attempt it without constitutional authority, the act would be a nullity, and could not be enforced.” 
This dovetails with the more general assertion made by supporters of the Constitution that any action taken by the federal government beyond its prescribed power would be void. Alexander Hamilton encapsulated this view in Federalist #78.
There is no position which depends on clearer principles, than that every act of a delegated authority contrary to the tenor of the commission under which it is exercised, is void. No legislative act, therefore, contrary to the constitution, can be valid.”
Nevertheless, many felt a bill of rights was vital, and agreed to ratification of the Constitution only on the condition of amendments. But that still left a glaring question unanswered: would the enumeration of rights in a bill or rights imply the federal government possessed the power to infringe on other rights not mentioned? After all, legal rule of construction dictates the “designation of one is the exclusion of the other.” So readers could logically interpret an enumeration of certain rights to exclude those not listed.”
James Wilson of Pennsylvania provided perhaps the most succinct explanation of the problem.
In all societies, there are many powers and rights which cannot be particularly enumerated. A bill of rights annexed to a constitution is an enumeration of the powers reserved [to the people]. If we attempt an enumeration, every thing that is not enumerated is presumed to be given. The consequence is, that an imperfect enumeration would throw all implied power into the scale of the government, and the rights of the people would be rendered incomplete.”
James Iredell of North Carolina made the same argument in the North Carolina ratifying convention, emphasizing that the Constitution, as written, already limited the federal government only to the powers delegated, and that the exercise of any powers beyond those given would constitute usurpation.
But when it is evident that the exercise of any power not given up would be a usurpation, it would be not only useless, but dangerous, to enumerate a number of rights which are not intended to be given up; because it would be implying, in the strongest manner, that every right not included in the exception might be impaired by the government without usurpation; and it would be impossible to enumerate every one. Let anyone make what collection or enumeration of rights he pleases, I will immediately mention 20 or 30 more rights not contained in it.” [Emphasis added]
So, supporters of a Bill of Rights faced a dilemma. They believed it was important to add further declaratory and restrictive clauses to the Constitution in order to prevent misconstruction or abuse of its powers. But they recognized it was impossible to list all of the rights and privileges the federal government was barred from interfering with.
Madison addressed this concern in a letter to Thomas Jefferson dated Oct. 17, 1788.
My own opinion has always been in favor of a bill of rights; provided it be so framed as not to imply powers not meant to be included in the enumeration.”
The Ninth Amendment provides a solution to this problem. It creates an exception to the accepted rule of construction and explicitly asserts that the federal government may only act within its delegated powers, both in regard to the rights listed in the Bill of Rights and those that were not listed.
 The Debates in the Several State Conventions on the Adoption of the Federal Constitution by Jonathan Elliot