MIT's 14.12 Economic Applications of Game Theory continues with instructor Ian Ball covering revenue equivalence, a core result in auction theory. Ball lays out the conditions under which different auction formats, such as first-price and second-price sealed-bid auctions, yield the same expected revenue for the seller despite awarding the good through different rules. The lecture builds the argument formally, working through the assumptions on bidder valuations and strategies needed for the equivalence to hold, and discusses why the result matters for comparing mechanisms that look procedurally different but produce identical outcomes in expectation. Running eighty minutes, this is lecture 19 in the fall 2025 course, delivered in MIT's standard blackboard and problem-driven style. It assumes familiarity with earlier game theory and mechanism design material from the course.