Educational only, not financial advice. Market rules and thresholds can change — verify current specifics with the exchanges or your broker.

Limit up and limit down are the upper and lower price bands that, when breached, trigger a volatility pause in a security — the mechanism behind LULD. Here’s how the bands work.

How the bands are set

The bands are a percentage above and below a rolling reference price — tighter for liquid, higher-priced (Tier 1) securities, wider for others — and they update continuously through the day. “Limit up” is the ceiling; “limit down” is the floor. Trades can occur within the band freely; the band constrains how far price can jump in one move.

What happens at the edges

If price reaches a band, it enters a “limit state” — it can trade at the band but not through it. If it doesn’t move back within the band within ~15 seconds, a 5-minute halt triggers, then trading reopens via auction (often at a new price). The bands also widen at the open and close, when volatility is naturally higher.

Limit up/down are the walls of the price band: price can bounce off them, but pushing through forces a pause to re-find fair value.

What it means for a scalper

For SPY, band breaches are rare (it’s enormously liquid and the S&P bands are wide). But the concept matters on extreme days and for individual stocks. It’s part of the volatility guardrail system alongside market-wide circuit breakers and the short-sale restriction.