Abstract: Most principals have a choice over the market design through which they transact---think public procurement, natural resource allocation, , or simple financial trades. Theory provides rich predictions on which designs strategic principals would or ought to choose under varying circumstances. Yet, little empirical evidence exists on how such principals choose among market designs, perhaps due to scarcity of appropriate settings. We empirically examine the principals' choice among sequential versus simultaneous and single versus package auctions using proprietary data from a particularly well-suited setting---the electronic market for corporate bonds. In this market, trades are numerous, all participants are sophisticated, and there are no exogenous frictions that impede their market design choices. [I will present a few early results, including that most traders choose simultaneous multi-item auctions with single bidding.