Article I, Section 9, Clause 6:
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.
The no-preference clause was designed to prevent preferences between ports because of their location in different states. Discriminations between individual ports are not prohibited. Acting under the Commerce Clause, Congress may do many things that benefit particular ports and that incidentally result to the disadvantage of other ports in the same or neighboring states. It may establish ports of entry, erect and operate lighthouses, improve rivers and harbors, and provide structures for the convenient and economical handling of traffic. 1 A rate order of the Interstate Commerce Commission that allowed an additional charge to be made for ferrying traffic across the Mississippi to cities on the east bank of the river was sustained over the objection that it gave an unconstitutional preference to ports in Texas. 2 Although there were a few early intimations that this clause was applicable to the states as well as to Congress, 3 the Supreme Court declared emphatically in 1886 that state legislation was unaffected by it. 4 After more than a century, the Court confirmed, over the objection that this clause was offended, the power that the First Congress had exercised 5 in sanctioning the continued supervision and regulation of pilots by the states. 6