*358 Mr. Justice White,

after stating the case, delivered the opinion of the court.

The right which the relator asserts rests upon the premise that the third section of the act of 1877 contractually dedicated the surplus fund of any year to tire payment of creditors holding claims for years subsequent to 1877, which claims were made by law payable out of the revenues for such subsequent years. From this is deduced the conclusion that the city charter, (sections 65 and 66 of the act of 1882,) and the amendment thereof in 1886, (act 109 of 1886,) which authorize the surplus in any year to be applied to -works of public improvement, are void so far as creditors subsequent to 1877 aré concerned, because they impair the obligations of the contract made in favor of such creditors by the act of 1877. The premise is fallacious and the conclusion drawn from it unsound. The act of 1877, after dedicating the revenues of each year to the expenses of that year, took any surplus out of the imperative rule thus established by the proviso that “ any surplus of said revenues may be applied to the indebtedness of former years.” In other words, having fixed inflexibly the rule by which the revenues of the year were to be first used to pay the debts of the year, it made an exception by allowing the' surplus of any year to be applied to the debts “of former years.” The rule was imperative; the exception permissive or facultative. Both provisions taken together operated to deprive the city government of power to use the revenues of one year to pay the debts of another, and to confer on the city, authority to employ, if it so chose, the surplus of one year to pay debts of previous years. Indeed, the law made no attempt to dedicate the surplus to any particular object or to control the legislative discretion of the municipal council in its regard. Having aifirmatively directed that the revenues of each year should be applied to the year’s expenses or debts, the surplus necessarily became subject to the appropriating power of the city. To prevent the general limitation dedicating each year’s revenues to each year’s debts, from operating to prevent the surplus from being applied to debts of previous years, should the city so desire, the law said the city “ may ” so use it.

*359 It is familiar doctrine that where a statute confers a power to be exercised for the benefit of the public or of a private person, the word “may” is often treated as imposing a duty rather than conferring a discretion. Mason v. Fearson, 9 How. 248; Washington v. Pratt, 8 Wheat. 681; Supervisors v. United States, 4 Wall. 435. This rule of construction is, however, by no means invariable. Its application depends on the context of the statute, and on wfiether it is fairly to be presumed that it was the intention of the legislature to confer a discretionary power or to impose an imperative duty. Minor v. Mechanics' Bank, 1 Pet. 46; Binney v. Chesapeake & Ohio Canal Co., 8 Pet. 201; Thompson v. Carroll's Lessee, 22 How. 422. In Minor v. Mechanics' Bank, Mr. Justice Story, delivering the opinion of the court, said (p. 63): “ The argument of the defendants is that ‘ may ’ in this section means ‘ must; ’ and reliance is placed upon a well-known rule, in the construction of public statutes, where the wmrd ‘may’ is often const-rued as imperative. Without question such a construction is proper in all cases where the legislature means to impose a positive and absolute duty, and not merely to give a discretionary power. But no general rule can be laid down upon this subject further than that that exposition ought to be adopted in this, as in other cases, which carries into effect the true intent and object of the legislature in the enactment. The ordinary meaning of the language must be presumed to be intended unless it would manifestly defeat the object of the provisions.”

In Thompson v. Lessee of Carroll, supra, this court, speaking through Mr. Justice Grier, observed (p. 434): “It is only where it is necessary to give effect to the clear policy and intention of the legislature that such a liberty can be taken with the plain words of the statute.”

In the law to be construed here it is evident that the word “ may ” is used in special contradistinction to the word “ shall,” and hence there can be no reason for “ taking such a liberty.” The legislature first imposes an imperative duty, the application of the revenue of each year to the expenses thereof, and then makes provision for the case of an excess of revenue *360 over expenses. In the first the word “ shall ” and in the latter provision the word “may” is used, indicating command in the one and permission in the other. Indeed, the discretionary nature of the power lodged in the city by the act of 1877, in regard to the surplus revenue of any year, results inevitably from the entire context of the statute and its obvious purpose. Under the'general rule which the statute created all the revenues of each year were to be applied exclusively to the expenditures of such year, hence they could not be used for any other purpose. If, after the expenses of any year had been paid out of its revenues, a balance remained on hand, the city would have been powerless to use it. She could not have applied, it to the payment of a debt, because the statute said that it should be devoted to the expenditures of -the year in which it was collected. She could not have applied it to the expenses of other years, for this, likewise, would have been a violation of the statute. She would simply have had in her possession a sum of money which she could not lawfully use for any purpose whatever. This condition of' things rendered it necessary to give power to dispose of the surplus; hence the use of the word “ may,” which clearly expresses this legislative intent.

The surplus having been .left by the act of 1877. under the control of the city council, it follows that that act gave- to the relator no contract right to such surplus. The city, having power to dispose of it, the acts of 1882 and 1886, directing the municipality to appropriate the surplus to works of public improvement, impaired the obligation of no contract right in favor of relator, since no right existed, and was therefore’, quoad the questions presented by this record, a valid exercise of legislative authority.

' Indeed, the necessary effect of granting the relief here sought would be.to impair the contract rights of creditors who are not before us. The record shows that under the mandatory terms of the statutes of 1882 and 1886 the surplus for the years covered by relator’s claim has been set apart to works of public improvement, and appropriations to that end have been made against the same. To make the mandamus peremp *361 tory would therefore take the fund from the creditors, to the payment of whose claims it has been lawfully consecrated, and give it to the relator.

The judgments in favor of the relator in no way change the situation. The first three direct “said judgment to be paid exclusively out of such revenues ... of the year 1882 . . .• and for which appropriations are made. in said amended budget, provided that any surplus of the revenues of any subsequent year may be applied to the'payment of the debts of the year 1882,.according to section 3 of act No. 30 of 1877.” The last fourteen, after providing that they should be paid out of the funds of the respective years, add, “ with the full benefit of the provisions of. section 3 of act No. 30 of 1877.” The proviso in all these judgments adds nothing to the rights conferred by the act of 1877, but in terms simply preserves them. What the position of the relator under that act is we have just stated. The manifest purpose of the saving clause in. the judgments was to prevent the language, which directed that they should be paid out of the funds of the year, from being construed as preventing the city government from paying out of the surplus, if so determined by the municipal authorities..

Judgment affirmed.