In January, the SEC approved the Twelfth Amendment to the Limit Up-Limit Down plan, as well as a handful of separate rule filings that will further harmonize how the exchanges handle a LULD trading pause. The centerpiece of these changes is a new framework for the exchanges’ reopening auction collars. In this piece, we give some context around the Twelfth Amendment and illustrate how the new collars will work.

In this document, we explain the origins of LULD, catalogue some of the important events that have shaped its evolution, and opine on the various proposals to improve it. At the end, we illustrate how LULD works in a two-page cheat sheet.