What is the Short Sale Restriction (SSR)? Definition, Formula, and Example
The Short Sale Restriction (SSR), formally SEC Rule 201, bars short sales at or below the national best bid for the rest of the trading day plus the next full session whenever a stock drops 10% or more from the prior close.
Plain-English Definition
The Short Sale Restriction (SSR) — formally Rule 201 of Regulation SHO — is the SEC's "alternative uptick rule." When a U.S.-listed stock declines 10% or more from the prior day's closing price, SSR triggers automatically, and for the remainder of that trading day plus the entire next trading session, short sales must be executed at a price strictly above the national best bid (NBB). The rule was adopted by the SEC in 2010 in response to the 2008 financial crisis and replaces the original 1938 uptick rule.
How SSR Is Triggered
SSR triggers on a single condition:
Trigger: Last trade price ≤ 0.90 × (prior session's official closing price)
Once triggered:
- A flag is set on the security in the consolidated SIP feed.
- Short-sale orders priced at or below the NBB are rejected by the executing venue.
- Short sales priced strictly above the NBB are permitted.
- The restriction lasts the rest of that day plus the full next trading day.
The "official closing price" is the consolidated last sale at 4:00 pm ET (or the auction close on listing exchanges). After-hours moves do not retrigger SSR; only the regular-session 10% threshold counts.
Worked Example
On May 2, 2024, SMCI closed at $738.06. The next morning, weak earnings drove the stock to $663 in pre-market and it gapped through the 10% threshold ($664.25) at the 9:30 am open. SSR triggered immediately. For the rest of May 3 and the entire May 6 session, any short order routed at or below the NBB was rejected. Traders who wanted to short had to post liquidity above the bid — meaning they sold to incoming buyers rather than hitting bids. SMCI continued lower into the close at $617, but the rule materially slowed the descent intraday by removing the most aggressive short-sale execution path.
When Traders Use It
SSR is a market-structure signal, not an indicator. Active traders watch the SSR flag (visible in Tapeboard's quote panel as "SSR: ON") for three reasons: (1) confirming a stock has crossed the 10% loss threshold without checking the chart, (2) calibrating short-side execution — market orders to short are effectively unavailable while SSR is active, and (3) gauging short-cover dynamics, since trapped shorts face higher slippage when SSR forces them to chase asks rather than hit bids when adding. Day traders treating SSR as a contrarian signal historically over-perform on next-day reversals when SSR triggers on news rather than technical breakdowns.
Limitations and Common Misconceptions
SSR does not ban short selling — it only restricts the execution price. Existing short positions can be held, covered, or rolled. The rule also does not apply to bona fide market makers performing two-sided quoting under the market-maker exception, nor to short sales that are part of a riskless principal transaction. The most common misconception is that SSR is the "uptick rule"; the original 1938 uptick rule required short sales above the last trade, while the 2010 rule references the NBB. SSR also does not prevent further declines — its empirical effect on intraday volatility is small (Diether et al., 2009), and stocks under SSR routinely keep falling.