In the old days — whether they were good or bad depends on your perspective — nothing like the May 6 follies could have happened to a stock listed on the New York Stock Exchange. Confronted with a large order imbalance, the Big Board halted trading and tried to sort it out. That might not prevent a stock from plunging, of course. But the delay gave time for cooler heads to leap in if they wished to do so.

In those days, the exchange had a monopoly on trading in stocks it listed. So it could delay without costing it any business.

Also in those days, commission costs were far higher than they are now. Big brokerage firms made markets in stocks for institutional investors, and charged them for it through commissions and, if the order was large or hard to place, through price discounts.

An institution that wanted to unload a large block of stock would call one of a handful of brokerage firms. If it wanted to get out fast, it knew that Goldman Sachs or Salomon Brothers or Morgan Stanley would agree to buy it without lining up customers to take the stock.

How big a discount the broker demanded would depend both on its assessment of the market and on the institution’s reputation. If an institution was known for sandbagging brokers, perhaps by selling on news that was not yet public, or by selling blocks simultaneously to several brokers, it would find that subsequent sales were much harder to pull off.

Now we have competition between exchanges and computers with programs aimed at allowing an institution to parcel out stock in small blocks, so no one knows it is unloading. Liquidity is largely provided by so-called high-frequency traders, who have their own superfast computers and models that are supposed to allow them to get in and out rapidly, making a small profit on almost every trade.