R2d § 58

Element: an acceptance must comply with the requirements of the offer as to the promise to be made or the performance to be rendered. This is the common-law mirror image rule.

Flender v. Tippins operationalizes the rule's collision with modern commerce: exchanged forms with conflicting forum clauses force the court into UCC § 2-207 territory and the knock-out rule.

Common misunderstanding: students think § 58 still controls modern dealmaking. For goods, UCC § 2-207 displaces it almost entirely. § 58 survives in services contracts and in the "last shot" rule when performance follows non-matching forms outside the UCC. Frame § 58 as the rule that 2-207 was a takanah to fix.