delivered the opinion of the Court.
We adhere to our decision rendered in the opinion filed after the first argument. 325 U. S. 711.1 That opinion *663expressly refrained from undertaking to make a definitive statement of what might be sufficient evidence of the collective agent’s authority either to settle finally the aggrieved individual employee’s claims or to represent him exclusively before the Adjustment Board. We do not attempt to do so now. For whether the collective agent has such authority is a question which may arise in many types of situations involving the grievances either of members of the union or of nonmembers, or both, and necessarily therefore no all-inclusive rule can be formulated for all such situations. But neither does this mean that an equally all-exclusive rule must be followed, namely, that authority can be given or shown only in some particular way.
The question whether the collective agent has authority, in the two pertinent respects, does not turn on technical agency rules such as apply in the simple, individualistic situation where P deals with T through A about the sale of Blackacre. We are dealing here with problems in a specialized field, with a long background of custom and practice in the railroad world. And the fact that § 3 First (i) provides that disputes between carriers and their employees arising out of grievances or out of the interpretation or application of agreements concerning rates of pay, rules or working conditions “snail be handled in the usual manner” up to and including the chief operating officer of the carrier, indicates that custom and usage may be as adequate a basis of authority as a more formal authorization for the union, which receives a grievance from an employee for handling, to represent him in settling it or in proceedings before the Board for its determination.2
*664Moreover, when an award of the Adjustment Board involving an employee’s individual grievance is challenged in the courts, one who would upset it carries the burden of showing that it was wrong.* *3 Its action in adjusting an individual employee’s grievance at the instance of the collective bargaining agent is entitled to presumptive weight. For, in the first place, there can be no presumption either that the union submitting the dispute would undertake to usurp the aggrieved employee’s right to participate in the proceedings by other representation of his own choice, or that the Board knowingly would act in disregard or violation of that right. Its duty, and the union’s, are to the contrary under the Act.4
Furthermore, the Board is acquainted with established procedures, customs and usages in the railway labor world. It is the specialized agency selected to adjust these controversies. ' Its expertise is adapted not only to interpreting a collective bargaining agreement,5 but also to ascer*665taining the scope of the collective agent’s authority beyond what the Act itself confers, in view of the extent to which this also may be affected by custom and usage.
We also pointed out that the Act imposes correlative affirmative duties upon the carrier, the collective agent and the aggrieved employee to make every reasonable effort to settle the dispute.* ****6 It would be entirely inconsistent for the Act to require the carrier and the union to negotiate concerning the settlement of the grievance and, while withholding power from them to make that settlement effective finally as against the employee, to reheve him altogether of obligation in the matter. Not only is he required to take affirmative steps. His failure to do so may result in loss of his rights.7
It is not likely that workingmen having grievances will be ignorant in many cases either of negotiations conducted between the collective agent and the carrier for their settlement or of the fact that the dispute has been submitted *666by one or the other to the Adjustment Board for determination. Those negotiations, as the Act requires, are conducted on the property. § 2 Sixth. Ordinarily submissions are not, and the statute contemplates that they shall not be, made to the Board until after all reasonable efforts to reach an agreement have been exhausted in good faith.8
In view of these facts there cannot be many instances in which an aggrieved employee will not have knowledge or notice that negotiations affecting his claim are being conducted or, if they fail, that proceedings are pending before the Board to dispose of it.® Although under our ruling his rights to have voice in the settlement are preserved, whether by conferring with the carrier and, having seasonably done so, refusing to be bound by a settlement reached over his' protest, or by having representation before the Board according to his own choice, we did not rule, and there is no basis for assuming we did, that an employee can stand by with knowledge or notice of what *667is going on with reference to his claim, either between the carrier and the union on the property, or before the Board on their submission, allow matters to be thrashed out to a conclusion by one method or the other, and then come in for the first time to assert his individual rights. No such ruling was necessary for their preservation and none was intended.
It may be, as we said previously, that respondents upon the further hearing will find it difficult to sustain their allegations, whether with reference to knowledge or notice in the material respects concerning which they have denied having it or otherwise. But whether this burden will be easy or impossible to carry, they are entitled to undertake it in the forum where such issues properly are triable.
The judgment is affirmed and the cause is remanded for further proceedings consistent with this opinion and the previous opinion filed in this cause.
Me. Justice Jackson took no part in the consideration or decision of this case on the reargument.The petition for rehearing, which resulted in setting the case for reargument, was supported by motions filed amicus curiae by various labor organizations and by the office of the Solicitor General. Upon granting of the motions, those organizations and the Solicitor General filed briefs amicus curiae and the latter participated in the argument. Various positions were taken upon the merits which we have considered but do not find it necessary to set forth.
Furthermore, so far as union members are concerned, and they are the only persons involved as respondents in this cause, it is altogether possible for the union to secure authority in these respects within well established rules relating to unincorporated organizations and their relations with their members, by appropriate provisions in *664their by-laws, constitution or other governing regulations, as well as by usage or custom. There was nothing to the contrary in our former opinion. We only ruled that on the showing made in this respect, which included controverted issues concerning the meaning and applicability of the union’s regulations, and the effects of custom and usage, we could not say as a matter of law that the disputed authority had been given.
In a somewhat different connection, which however we think not without weight here, § 3 First (p) provides that the Board’s award "shall be prima facie evidence of the facts therein stated” in the statutory suit provided for enforcement of awards.
The contrary practice noted in our former opinion, 325 U. S. 732-733, has been due without question, we think, to the Board’s erroneous conception, accepted generally also by the unions and strongly urged in this case especially upon the reargument, that the Act itself, notwithstanding the provisions particularly of § 3 First (j) and the proviso to § 2 Fourth, confers exclusive statutory power upon the collective agent to deal with the carrier concerning individual grievances and to represent the aggrieved employee in Board proceedings.
We recently emphasized this in Order of Railway Conductors v. Pitney, 326 U. S. 561, 567, in which we said: “Since all parties seek *665to support their particular interpretation of these agreements by evidence as to usage,' practice and custom, that too must be taken into account and properly understood. The factual question is intricate and technical. An agency especially competent and specifically designated to deal with it has been created by Congress.”
See 325 U. S. 711 at notes 12,18 and text. We said: “The obligation [to negotiate] is not partial. In plain terms the duty is laid on carrier and employees alike, together with their representatives; and in equally plain terms it applies to all disputes covered by the Act, whether major or minor.” Note 18. Cf. Virginian R. Co. v. System Federation, 300 U. S. 515, 548; Railroad Trainmen v. Toledo, P. & W. R. Co., 321 U. S. 50, 56 ff.
Even the ordinary law of agency attributes authority to a representative to act when the principal stands by with knowledge or notice of his assumption of that authority and permits the third person to act to his injury upon the same assumption. Cf. Seavey, The Rationale of Agency (1920) 29 Yale L. J. 859, 873 et seq.; and other authorities cited in Mechem, Cases on the Law of .Agency (3d ed.) 186, note. And of course the assumption that even so-called common law rules of agency allow no room for the play of usage and custom is, to say the least, naive.
Cf. note 6 and authorities cited.
We pointed out in the former opinion that § 3 First (j) expressly provides that “the several divisions of the Adjustment Board shall give due notice of all hearings to the employee or employees and the carrier or carriers involved in any dispute submitted to them,” 325 U. S. at 731, 734, and this provision, with the emphasis we placed upon the phrase “to the employee” and the conjunction of the provision for “due notice” with the provision for representation “in person, by counsel, or by other representatives,” was one of the statutory mainstays for our conclusion that the Act did not give -the collective agent the exclusive powers over the settlement of grievances claimed for it.
But we did not undertake to define what was meant by “due notice,” nor do we now. “Due notice” conceivably could be given or had in a variety of forms, more especially when account is taken of the generally informal procedure of the Board. It would require at the least, we think, knowledge on the aggrieved employee’s part of the pendency of the proceedings or knowledge of such facts as would be sufficient to put him on notice of their pendency.