Rule 15c3-1 measures a broker's net capital: shareholders' equity, minus assets that cannot be turned into cash quickly, minus percentage haircuts on securities positions that reflect how far the price might move before they are sold. The result must exceed a minimum tied to the firm's business, which is far larger for a firm that carries customer accounts than for one that introduces them elsewhere.
The rule is a liquidation standard, not a going-concern one. The question it asks is whether the firm could wind down and return customer property without outside money. That is why proprietary trading inventory is discounted heavily and why illiquid receivables count for nothing.
Firms must notify regulators when capital falls toward early-warning thresholds, and trading is restricted below them. A broker in net capital trouble typically stops accepting new positions long before it fails, which is one of the earliest public signs of stress.
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