An ancillary contract is a contract in which the parties to a contract enter into another contract or promise to enter into another contract. The two treaties are therefore linked and can be applied, although they do not constitute a constructive element of the original treaty.  In JJ Savage and Sons Pty Ltd v. Blakney, a mere expression of opinion was found to be insufficient to be kept as a promise. In Crown Melbourne Limited v Cosmopolitan Hotel (Vic) Pty Ltd, a statement by an owner to tenants, when negotiating a lease agreement that they would be “taken over at the extension period”, would not require the lessor to offer another five-year lease.  Primary and secondary contracts are active simultaneously and, in some cases, the provisions of the latter may prevail over those of the former. For example, companies X and Y conclude a construction contract with X as the client and Y as the contracting authority. Y then concludes a warranty contract with Z, a hardware supplier. If the materials are found to be defective, X Z can sue even though they do not have a contract with each other. This rule prevents the parties from altering the importance of written contracts by oral or tacit agreements that are not included in the original contract and thus undermining its integrity.
In other words, if a contract is concluded in writing, subsequent agreements that are not concluded in writing are not used as evidence in a contractual dispute. There are, however, several exceptions to this rule. In the case of a bilateral guarantee contract, the two parties concluding the main contract also enter into the guarantee contract. A tripartite security agreement contains an obligation of a third party that is not a party to the original contract. This is often used, for example, in the case of a sales contract. The promisor must have explicitly or implicitly requested the main contract and his conviction must be for the purpose of obtaining the entry of the other party into the main contract.  According to Lord Denning MR, an ancillary contract is considered binding “when one person makes a promise or insurance to another, with the intention of reacting by entering into a contract”.  An ancillary contract, when concluded between the same parties as the main contract, cannot be contrary to the main contract. In other words, if the duration was agreed before the conclusion of the formal contract (but it was still included and could not be performed until the end of the second period), the first period remains eligible.  In essence, warranty contracts cannot contradict any element of the main contract or the rights it creates.  It can also be embodied as follows: a security agreement is a contract that induces a person to enter into a separate “primary” contract. .