On April 16, 2010, the Securities and Exchange Commission (SEC) filed a civil complaint against Goldman Sachs in the U.S. District Court for the Southern District of New York. The complaint alleged that Goldman Sachs violated the anti-fraud provisions of the federal securities laws in connection with a 2007 synthetic collateralized debt obligation (CDO) transaction, ABACUS 2007-AC1 SPV (ABACUS). Goldman Sachs agreed to a $550 million settlement with the SEC on July 15, 2010. We analyze the ABACUS transaction and the SEC's complaint against Goldman Sachs in the context of recent technological changes within the investment banking market. Investment banking was historically a relationship-based business, sustained by reputationally intermediated tacit contracts. Recent advances in information technology and financial economics have codified many formerly tacit elements of investment banking. As a result, some investment banking deals are now transacted at arm's length and rely more upon formal contracts; we argue that, for this type of deal, there is a stronger case for legal rules regulating the investment bank counterparty relationship. However, some deals continue to be arbitrated by tacit rules and norms and, for these deals, legal rules are less appropriate, because it is very hard for a third party to ascertain tacit understandings made in the context of a long-lived relationship. An attempt to introduce legal rules into reputationally intermediated relationships may even impair the counterparties' ability to arrive at informal arrangements, and so to trade. The supervision of deals like ABACUS should therefore reflect the extent to which they are transactional or relational; we argue that in neither case is there justification for the application of legal rules or the gap-filling standard of fiduciary duties.