What market safeguard became common after the 1987 crash to pause trading during steep falls?

The market safeguard that became common after the 1987 crash was circuit breakers.

Circuit breakers are automatic or discretionary trading halts triggered by unusually large market declines. Their purpose is to interrupt panic-driven selling, give exchanges and clearing systems time to process orders, and let investors absorb new information before trading resumes.

The October 19, 1987, collapse exposed weaknesses in the coordination of stock, options, and futures markets. The Brady Commission recommended linked safeguards, and U.S. exchanges introduced the first modern market-wide circuit-breaker rules in 1988. The mechanisms have since been revised several times.

Circuit breakers are not the same as margin calls, which demand additional collateral from leveraged traders. Tender offers are proposals to buy shares, while short squeezes occur when rising prices force short sellers to buy. Modern U.S. thresholds are based on percentage declines in the S&P 500, rather than the old point-based system tied to the Dow.