The legal concept of privity of contract, which dictates that generally, only the parties who agree to a contract possess the rights and obligations under it. Historically, this rule strictly prevented third parties from enforcing contractual terms, but the text details several significant exceptions that have evolved to address potential injustices, such as the Contracts (Rights of Third Parties) Act 1999, collateral contracts, and trusts. Furthermore, it extensively covers the legal framework of agency, illustrating how an agent can contractually bind a principal with third parties through different forms of authority, specifically actual authority (express or implied) and apparent authority. These exceptions and the principles of agency outline various circumstances where individuals or entities not originally party to an agreement can still acquire rights or be affected by the contract's terms.