In English-derived common law legal systems (such as that of the USA), the required elements are:

• Agreement:
  • Offer. This entails:
    • Quantity (what is being exchanged)
    • Time (when the contract must be performed)
    • Identification of parties
    • Price
    • Subject matter (what is the person making the offer willing to give) (Additionally, there must be serious intent to enter into a bargain, and certainty and definiteness of terms.)
  • Acceptance. This entails:
    • Serious intent to be bound.
    • Communication to offeror.
  (Offer and acceptance jointly establish privity of contract.)
• Consideration.
• Capacity (of offeror and offeree).
• Lawful purpose.
• Genuineness of assent (no fraud, duress, undue influence).
• Form (i.e., some kinds of contract must be of written form).

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