This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Performance of a contract by one party may be made wholly or in part conditional upon performance by another. Thus a contract by which A promises to render certain services, and X agrees to pay for them, may be so worded that X will not be called upon to pay unless A has performed the services, or, on the other hand, it may be so arranged that A will not be obliged to act unless he is paid in advance. So if it is provided in a contract that X is to pay for A's services only if they were completed by a certain day, and A does not complete his services by that time, A cannot, unless X had lost the right to insist upon the condition, recover under the contract. His only ground of recovery would be under an acceptance of the services by X, and a consequent implied contract. Not only may such conditions be expressed, but they may also be implied by the law. It is therefore necessary for a party to a contract to consider carefully whether he has done everything which he is called upon to do before he can maintain an action against the other party for the failure of that other party to perform his obligation under the contract. When by the contract performance by the one is expressly made conditional upon performance by the other, the case is clear; if no such conditions are expressed, the question arises whether any are implied. A court in construing a contract in this respect, as in others, will attempt to ascertain the intention of the parties, and if it appears to have been intended that the whole or a part of performance by one party was meant to be dependent upon some portion of the performance by the other party, then effect will be given to this intention by holding performance under the contract conditional according to the intention shown. Take for instance a contract for the sale of land: one party may agree to convey the land at a certain time, and the other to pay the purchase money at the same time, without expressly saying that either act should be dependent upon the performance of the other. Yet, as the meaning is clear that the acts are in reality to be mutually dependent, neither party is called upon to perform unless the other party is ready, able, and willing to carry out his part; and, on the other hand, neither party can maintain an action for breach without showing himself to have been able, ready and willing to perform on his side. The practical result of these principles is that when B breaks his contract with A, and A wishes to hold B liable for the breach, A must carefully consider whether he has done all on his part that is necessary to put B in the wrong. As questions of some nicety occur on such points, it is desirable to take advice of counsel in season to follow out any suggestions regarding such steps as a pre-liminary to a suit.