MR. JUSTICE JACKSON delivered the opinion of the Court.

A decision of the Supreme Court of Pennsylvania has deprived petitioners of an injunction which a lower equity court of the State had granted to prohibit certain picketing by respondent labor union. The court below reviewed the national Labor Management Relations Act and our applicable decisions, and concluded: "In our opinion such provisions for a comprehensive remedy precluded any State action by way of a different or additional remedy for the correction of the identical grievance." The correctness of this ruling is the sole issue here. We granted certiorari.

Petitioners were engaged in the trucking business and had twenty-four employees, four of whom were members of respondent union. The trucking operations formed a link to an interstate railroad. No controversy, labor dispute or strike was in progress, and at no time had petitioners objected to their employees joining the union. Respondents, however, placed rotating pickets, two at a time, at petitioners' loading platform. None were employees of petitioners. They carried signs reading "Local 776 Teamsters Union (A. F. of L.) wants Employees of Central Storage Transfer Co. to join them to gain union wages, hours and working conditions." Picketing was orderly and peaceful, but drivers for other carriers refused to cross this picket line and, as most of petitioners' interchange of freight was with unionized concerns, their business fell off as much as 95%. The courts below found that respondents' purpose in picketing was to coerce petitioners into compelling or influencing their employees to join the union.

The equity court held that respondents' conduct violated the Pennsylvania Labor Relations Act. The Supreme Court of the Commonwealth held, quite correctly, we think, that petitioners' grievance fell within the jurisdiction of the National Labor Relations Board to prevent unfair labor practices. It therefore inferred that state remedies were precluded. The dissenting judge thought the federal remedy inadequate, as a practical matter, because the slow administrative processes of the National Labor Relations Board could not prevent imminent and irreparable damage to petitioners. Since our decisions have not specifically denied the power of state courts to enjoin such injury, he thought the injunction should be sustained.

The national Labor Management Relations Act, as we have before pointed out, leaves much to the states, though Congress has refrained from telling us how much. We must spell out from conflicting indications of congressional will the area in which state action is still permissible.

This is not an instance of injurious conduct which the National Labor Relations Board is without express power to prevent and which therefore either is "governable by the State or it is entirely ungoverned." In such cases we have declined to find an implied exclusion of state powers. International Union v. Wisconsin Board, 336 U.S. 245, 254. Nor is this a case of mass picketing, threatening of employees, obstructing streets and highways, or picketing homes. We have held that the state still may exercise "its historic powers over such traditionally local matters as public safety and order and the use of streets and highways." Allen-Bradley Local v. Wisconsin Board, 315 U.S. 740, 749. Nothing suggests that the activity enjoined threatened a probable breach of the state's peace or would call for extraordinary police measures by state or city authority. Nor is there any suggestion that respondents' plea of federal jurisdiction and pre-emption was frivolous and dilatory, or that the federal Board would decline to exercise its powers once its jurisdiction was invoked.

Congress has taken in hand this particular type of controversy where it affects interstate commerce. In language almost identical to parts of the Pennsylvania statute, it has forbidden labor unions to exert certain types of coercion on employees through the medium of the employer. It is not necessary or appropriate for us to surmise how the National Labor Relations Board might have decided this controversy had petitioners presented it to that body. The power and duty of primary decision lies with the Board, not with us. But it is clear that the Board was vested with power to entertain petitioners' grievance, to issue its own complaint against respondents and, pending final hearing, to seek from the United States District Court an injunction to prevent irreparable injury to petitioners while their case was being considered. The question then is whether the State, through its courts, may adjudge the same controversy and extend its own form of relief.

Congress did not merely lay down a substantive rule of law to be enforced by any tribunal competent to apply law generally to the parties. It went on to confide primary interpretation and application of its rules to a specific and specially constituted tribunal and prescribed a particular procedure for investigation, complaint and notice, and hearing and decision, including judicial relief pending a final administrative order. Congress evidently considered that centralized administration of specially designed procedures was necessary to obtain uniform application of its substantive rules and to avoid these diversities and conflicts likely to result from a variety of local procedures and attitudes toward labor controversies. Indeed, Pennsylvania passed a statute the same year as its labor relations Act reciting abuses of the injunction in labor litigations attributable more to procedure and usage than to substantive rules. A multiplicity of tribunals and a diversity of procedures are quite as apt to produce incompatible or conflicting adjudications as are different rules of substantive law. The same reasoning which prohibits federal courts from intervening in such cases, except by way of review or on application of the federal Board, precludes state courts from doing so. Cf. Myers v. Bethlehem Shipbuilding Corp., 303 U.S. 41; Amalgamated Utility Workers v. Consolidated Edison Co., 309 U.S. 261. And the reasons for excluding state administrative bodies from assuming control of matters expressly placed within the competence of the federal Board also exclude state courts from like action. Cf. Bethlehem Steel Co. v. New York Board, 330 U.S. 767.

This case would warrant little further discussion except for a persuasively presented argument that the National Labor Relations Board enforces only a public right on behalf of the public interest, while state equity powers are invoked by a private party to protect a private right. The public right, it is said, is so distinct and dissimilar from the private right that federal occupancy of one field does not debar a state from continuing to exercise its conventional equity powers over the other. Support for this view is accumulated from the Act itself, its legislative history, some judicial expression, and professional commentary.

It is true that the Act's preamble emphasizes the predominance of a public interest over private rights of either party to industrial strife, and declares its purpose to proscribe practices on the part of labor and management which are inimical to the general welfare, and to protect the rights of the public in connection with labor disputes affecting commerce. And some language of the Act seems to contemplate a remedy to supplement, rather than to substitute for, existing ones.

Also, the Senate Committee, reporting the bill, said:

"After a careful consideration of the evidence and proposals before us, the committee has concluded that five specific practices by labor organizations and their agents, affecting commerce, should be defined as unfair labor practices. Because of the nature of certain of these practices, especially jurisdictional disputes, and secondary boycotts and strikes for specifically defined objectives, the committee is convinced that additional procedures must be made available under the National Labor Relations Act in order adequately to protect the public welfare which is inextricably involved in labor disputes.

". . . Hence we have provided that the Board, acting in the public interest and not in vindication of purely private rights, may seek injunctive relief in the case of all types of unfair labor practices and that it shall also seek such relief in the case of strikes and boycotts defined as unfair labor practices . . . ."

We are also reminded that this Court, in Amalgamated Utility Workers v. Consolidated Edison Co., supra, at 265, recognized this distinction by saying, "The Board as a public agency acting in the public interest, not any private person or group, not any employee or group of employees, is chosen as the instrument to assure protection from the described unfair conduct in order to remove obstructions to interstate commerce." Various statements may also be cited in which the Board would appear to have recognized a distinction between public and private rights or interest in labor controversies.

It often is convenient to describe particular claims as invoking public or private rights, and this handy classification is doubtless valid for some purposes. But usually the real significance and legal consequence of each term will depend upon its context and the nature of the interests it is invoked to distinguish.

Statutes may be called public because the rights conferred are of general application, while laws known as private affect few or selected individuals or localities. Or public rights may mean those asserted by the state as a party either in criminal or civil proceedings. Again, the body of learning we call conflict of laws elsewhere is called private international law because it is applied to adjustment of private interests, while public international law is applicable to the relations between states. At other times, rights will be characterized by the body of law from which they are derived; but such distinction between public and private law is less sharp and significant in this country, where one system of law courts applies both, than in the Continental practice which administers public law through a system of courts separate from that which deals with private law questions. Perhaps in this country the most usual differentiation is between the legal rights or duties enforced through the administrative process and those left to enforcement on private initiative in the law courts.

Federal law has largely developed and expanded as public law in this latter sense. It consists of substituting federal statute law applied by administrative procedures in the public interest in the place of individual suits in courts to enforce common-law doctrines of private right. This evolution, sharply contested, and presenting many problems, has taken place in many other fields as well as in labor law. For example, the common law recognized a shipper's right to have a common carrier transport his goods for reasonable rates, and the right was enforceable in the courts. But this private right proved too costly and sporadic to be effective as transport became a vast enterprise. As to interstate commerce, this right was superseded by the Interstate Commerce Act, which, in the public interest, authorized a public tribunal to prescribe reasonable rates and to award reparations for excessive ones. Of course, this put an end to private litigation in state and federal courts to determine, in the first instance, what rate for carriage is reasonable, although that Act did not expressly abolish the pre-existing private rights.

Even if we were to accept as significant the distinction between public and private rights and regard the national Labor Management Relations Act as enforcing only public rights, the same reasoning would prevent us from assuming that the Pennsylvania labor statute declares rights of any different category. It is true that petitioners sought an injunction to restrain damage to their own business. But the injunction appears to have been granted because the picketing violated the state statute, and neither the statutory language nor the opinion of the Pennsylvania Supreme Court warrants a conclusion that the statute protects private rights, as most authorities would define the term. Passed in 1937, the statute recites that the growing inequality of bargaining power between employers and employees "substantially and adversely affects the general welfare of the State" and that certain practices tend to create "industrial strife and unrest, which are inimical to the public safety and welfare, and frequently endanger the public health." Encouragement of collective bargaining is declared "the public policy of the State." And one subsection reads: "This act shall be deemed an exercise of the police power of the Commonwealth of Pennsylvania for the protection of the public welfare, prosperity, health, and peace of the people of the Commonwealth."

This language is comparable, on the state level, to the language in the federal Act. If Congress was protecting a public, as opposed to a purely private, interest, the same could be said of the Pennsylvania Legislature. The State Supreme Court has not said otherwise. The court opinion, of course, did not analyze in detail the state law basis for injunction in this case because it found lack of state jurisdiction, and the dissenting opinion discussed the jurisdictional aspect of the case and did not reach the merits. But we find no basis at all for petitioners' argument that the equity courts, which in Pennsylvania enforce the labor relations statute, would enforce rights of any different category, or of any less public or more private character, than those enforced by the National Labor Relations Board.

Further, even if we were to assume, with petitioners, that distinctly private rights were enforced by the state authorities, it does not follow that the state and federal authorities may supplement each other in cases of this type. The conflict lies in remedies, not rights. The same picketing may injure both public and private rights. But when two separate remedies are brought to bear on the same activity, a conflict is imminent. It must be remembered that petitioners' state remedy was a suit for an injunction prohibiting the picketing. The federal Board, if it should find a violation of the national Labor Management Relations Act, would issue a cease-and-desist order and perhaps obtain a temporary injunction to preserve the status quo. Or if it found no violation, it would dismiss the complaint, thereby sanctioning the picketing. To avoid facing a conflict between the state and federal remedies, we would have to assume either that both authorities will always agree as to whether the picketing should continue, or that the State's temporary injunction will be dissolved as soon as the federal Board acts. But experience gives no assurance of either alternative, and there is no indication that the statute left it open for such conflicts to arise.

The detailed prescription of a procedure for restraint of specified types of picketing would seem to imply that other picketing is to be free of other methods and sources of restraint. For the policy of the national Labor Management Relations Act is not to condemn all picketing but only that ascertained by its prescribed processes to fall within its prohibitions. Otherwise, it is implicit in the Act that the public interest is served by freedom of labor to use the weapon of picketing. For a state to impinge on the area of labor combat designed to be free is quite as much an obstruction of federal policy as if the state were to declare picketing free for purposes or by methods which the federal Act prohibits.

Whatever purpose a classification of rights as public or private may serve, it is too unsettled and ambiguous to introduce into constitutional law as a dividing line between federal and state power or jurisdiction. Perhaps the clearest thing to emerge from the best-considered literature on this subject is that the two terms are not mutually exclusive, that the two classifications overlap, and that they are of little help in cases such as we have here. In those cases where this Court has employed the term, it has been chiefly as an aid in statutory construction. Cf. Federal Trade Commission v. Klesner, 280 U.S. 19.

Our decisions dealing with injunctions have been much concerned with the existence and nature of private property rights, but no case is cited or recalled in which this Court has recognized the distinction between private and public rights to reach such consequences as are urged here. Myers v. Bethlehem Shipbuilding Corp., 303 U.S. 41; Frost v. Corporation Commission, 278 U.S. 515; Cavanaugh v. Looney, 248 U.S. 453; International News Service v. Associated Press, 248 U.S. 215; In re Debs, 158 U.S. 564; In re Sawyer, 124 U.S. 200.

We conclude that when federal power constitutionally is exerted for the protection of public or private interests, or both, it becomes the supreme law of the land and cannot be curtailed, circumvented or extended by a state procedure merely because it will apply some doctrine of private right. To the extent that the private right may conflict with the public one, the former is superseded. To the extent that public interest is found to require official enforcement instead of private initiative, the latter will ordinarily be excluded. Of course, Congress, in enacting such legislation as we have here, can save alternative or supplemental state remedies by express terms, or by some clear implication, if it sees fit.

On the basis of the allegations, the petitioners could have presented this grievance to the National Labor Relations Board. The respondents were subject to being summoned before that body to justify their conduct. We think the grievance was not subject to litigation in the tribunals of the State.

Judgment affirmed.