The article focuses on the bond market in the United States. The New York Stock Exchange (NYSE) has begun talks with the Securities and Exchange Commission (SEC) over permission to trade thousands of unlisted corporate bonds. The obvious attraction of the bond market is its size: some $10 trillion-worth of corporate bonds traded in 2003, a quarter more than the trading volume on the NYSE. Two years ago, the National Association of Securities Dealers began collecting transaction prices. This month the SEC released a study, based on these data, by its own Amy Edwards and Michael Piwowar, and Lawrence Harris of the University of Southern California. For all but the largest investors, trading bonds is many times more expensive than trading stocks. This is odd. In theory bonds should be no more expensive to trade than equities. Even before sales commissions, the spread between what investors receive when selling $20,000-worth of corporate bonds and what they pay when buying is about 1.4%, or about one-quarter of a year's return. Costs are still higher in the convertible-bond market. And in the municipal-bond market, the spread reaches 2%, according to a similar study published by the SEC in the summer. All told, this is at least four times as expensive as a typical retail equity trade. The prices of most corporate bonds are reported soon after trading. But for the portion of the corporate-bond market where prices are still not required to be reported, a $2 trillion segment in 2003, the study reckons investors paid an additional $1 billion in trading costs. Bond dealers have long resisted efforts to shine a light on their markets. The study indicates why.