Independent technological glitches forced two separate trading halts on different U.S. exchanges during the week of July 6, 2015. During each halt, all other exchanges remained open. We exploit exogenous variation provided by this unprecedented coincidence, in conjunction with a proprietary data set, to identify the causal impact of Designated Market Maker (DMM) participation on liquidity. When the voluntary liquidity providers on one exchange were removed, liquidity remained unchanged; when DMMs were removed, liquidity decreased market-wide. We find evidence consistent with the idea that these DMMs, despite facing only mild formal obligations, significantly improve liquidity in the modern electronic marketplace.

Security trading now fragments into more than ten almost identical stock exchanges in the United States. We show that discrete pricing is one economic force that prevents the consolidation of trading volume. The uniform one-cent tick size (minimum price variation), imposed by the SEC's Rule 612, leads to more dispersed trading for lower priced securities. When a security reverse splits, its price increases and relative tick size (one cent divided by the price) decreases. We find that reverse splits consolidate trading of securities, using securities with identical underlying fundamentals that do not reverse split as the control group.