Article 1, Section 9, Clause 6: Prohibition on Port Preferences

No Preference shall be given by any Regulation of Commerce or Revenue to the Ports of one State over those of another: nor shall Vessels bound to, or from, one State, be obliged to enter, clear, or pay Duties in another.

This clause might seem uncontroversial, but in fact there have been controversies over whether Congress could (for example) fund improvements in one port that were not funded in all ports. The answer of the courts is yes, so long as it is individual ports that are given such preference and not because of their location in a particular state. The same follows for various kinds of regulations. This interpretation very nearly makes the clause a dead letter.