The full order type inventory

Sophomore · enrolled learners

This lesson opens with When the book went to 2 percent.

What happened

During the deleveraging event of 10 October 2025, analysis of the episode reported market depth collapsing by around 98 percent, from roughly $1.2M to about $27,000, as market makers withdrew. More than $19B of positions were forcibly closed within 24 hours across more than 1.6 million accounts. Every resting order in the market at that moment behaved exactly as specified: market orders filled against whatever remained, stop orders triggered and became market orders, limit orders sat unfilled or were skipped, and post-only orders were rejected. No order type malfunctioned and no exchange failed to honor its own rules.

The decision point

Order types are usually chosen by imagining normal conditions, in which the differences between them are small. Their behavior diverges enormously precisely when depth is gone, which is the only situation in which the choice matters, and it is the situation nobody is picturing when they select from the dropdown.

Orientation and Year One are open: anyone can read them without an account. From Year Two onward the lessons are for enrolled learners, because progress through the later years only means anything if it is tracked against a record.

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