Johannes Pfeifenberger and Judy Chang are principals of The Brattle Group. Delphine Hou is a former Brattle associate. This article is based on work undertaken for the Southwest Power Pool’s Regional State Committee and the associated report, Seams Cost Allocation: A Flexible Framework to Support Interregional Transmission Planning, April 2012, available at www.spp.org and www.brattle.com. The authors acknowledge sole responsibility for the content of this article.