Now that we have laid out the different types of participants in the US equities market and what their goals might be, we'll move on to discussing the logistics of communications between them.

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The primary concept of the "trading day" refers to the hours from 9:30 am to 4:00 pm, eastern standard time, Monday through Friday, except for certain declared market holidays.

Would-be buyers and sellers of securities on exchanges must advertise prices over $1 in whole pennies, and trades must be executed at whole penny increments with some notable exceptions.

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Once an investor has decided what they'd like to buy or sell, how much, and some goals in terms of price and/or timing, they need a way to communicate this to a broker.

There is a lot of information that flows back and forth between the trading venues and their members (the brokers and proprietary traders who are trading on their platform).

In the realm of retail trading, some communication happens directly between brokers and proprietary traders. In fact, most retail trading occurs through direct relationships between retail brokers and proprietary trading firms.

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Here we will discuss the main regulations that serve as constraints on market activity, and the communications and data that regulators rely upon to enable their enforcement.