National Market System (NMS) is a set of rules passed by the Securities and Exchange Commission (SEC), which looks to improve the U.S. exchanges through improved fairness in price execution as well as improve the displaying of quotes and amount and access to market data. Taobiz explains Regulation NMS This regulatory ruling is comprised of four main components: The Order Protection Rule aims to ensure that investors receive the best price when their order is executed by removing the ability to have orders traded through (executed at a worse price). The Access Rule, aims to improve access to quotations from trading centers in the National Market System by requiring greater linking and lower access fees. The Sub-Penny Rule, which sets the lowers quotation increment of all stocks over $1.00 per share to at least $0.01. Market Data Rules, which allocate revenue to self-regulator organizations that promote and improve market data access.