Whatever rights defendant secured are ascertainable by and referable to the written contract offered in evidence. The principle is elementary that its terms cannot be added to or suffer diminution by testimony *649of the previous understanding of the parties or of their acts subsequent thereto, where there is no ambiguity. Being in writing, it must be interpreted and construed according to the language used. It is not claimed that its terms were obscure or ambiguous. Hence it was clearly improper to permit evidence to be received of the oral understanding of the parties or as to their personal interpretation of the contract. The rules mentioned were violated by permitting one of defendant’s officers to testify as to Ms understanding of what he was to receive for the $200 which was offered in the proposal to the plaintiff. They were violated again when the defendant was allowed to introduce a letter written by plaintiff’s secretary a few days after the contract was made, giving his interpretation of it. It was for the court, and not the parties, to. ascertain what the contract was, and to construe it. Ranney v. Higby, 5 Wis. 62; Diefenback v. Stark, 56 Wis. 462; Farnsworth v. Brunquest, 36 Wis. 202.

The evidence clearly shows that the list of carnival events had been given to the newspapers and published before defendant made the proposition sued upon. That proposition contained no reference to an exclusive privilege of publishing the mere program of events. The right given by the contract was to issue a publication containing the official program, and gotten up in such form as would induce people to purchase it as a souvenir of the occasion. The fact that it was to be issued under the sanction of the Carnival Association, and designated as an “ official souvenir program,” was one the defendant, no doubt, anticipated would give it sale value. The features which were to give it the character of a souvenir were exclusively within the control of the defendant. It is not claimed that the mere list of events had any special commercial value. It was the fact that it had official sanction that was supposed to give it importance. In line with this idea, the defendant was permitted to intro*650duce in evidence a contract of the association with one Brown, in which the latter was authorized to issue a work under its auspices entitled “ Milwaukee and its Commercial Industries.” The publisher was to incorporate in it matter descriptive of the objects and events of the carnival, and no other publication of a similar nature was to be issued under the authority of the association, except the official program of events. The defendant was also permitted to put in evidence a copy of» the' publication so issued. It contained a write-up of the various commercial industries of the city, a description of the principal features of the proposed celebration, a list of the events as scheduled, and purported to be published “ under the auspices of the Carnival Association.” The court also permitted witnesses, both for plaintiff and defendant, to testify as to their interpretation of the contract. The court was undoubtedly wrong in this, regard. The court, and not the witnesses, should have construed its provisions, unless there was such doubt about it that he would have been warranted in submitting the matter to the jury under the rule mentioned in Vilas v. Bundy, 106 Wis. 168, 176, and cases cited. To us it seems plain that the defendant secured the right to publish the official program in such form and with such features as would give it the character of a token of remembrance, a souvenir, a memento, of the occasion. The use of the word “ souvenir ” indicates, that the publication was to be something more than a mere list of events. Being called “ official ” might, and probably did, give it additional significance; but that word alone is not of overwhelming weight. The whole contract must be construed together in the light of the project in view. As already suggested, it contemplated publication of the program of exercises in such form as would induce people to purchase it as a souvenir of the celebration. ¥e are unable to see how this right was infringed upon by the publication of the list of events in the so-called “ Brown book.” Excluding' *651the evidence improperly admitted, there was no evidence requiring a submission of the case to the jury. A verdict for plaintiff should have been directed.

The charge of the court permitted the jury- to determine not only what the contract really was, but also allowed them to determine how it it was understood by the parties. This was clearly erroneous. Under the contract in evidence it was the judicial function to ascertain and interpret it, and then allow the jury to say whethér it was breached.

By the Oourt.— Judgment reversed, and the cause is remanded for a new trial.