Price-Time Priority
The rule by which an exchange ranks orders for matching: better prices first, and among equal prices, whichever arrived earlier.
How it is identified
Rank = better price first; among equal prices, earlier timestamp first
Unit
qualitative
In depth
Price-time priority is why an exchange is fair in a specific, narrow sense: no participant can jump the queue by being larger or better connected, only by offering a better price or arriving sooner. The 'sooner' half is what makes latency valuable and funds the whole business of co-location. Modifying an order's price or increasing its quantity forfeits its time stamp and sends it to the back of that price level, while reducing quantity usually does not. Traders who repeatedly nudge a limit order are often destroying the queue position they were waiting on.
Worked example
Three orders rest at ₹250: 1,000 shares from 09:20, 2,000 from 09:31 and 500 from 09:45. A sell of 1,500 fills the 09:20 order completely and 500 of the 09:31 order. The 09:45 order is untouched despite the same price.
Illustrative figures, chosen so the arithmetic is easy to follow. Not a live price and not a valuation of any company.
Educational reference only
This entry explains what “Price-Time Priority” means. It is not investment advice and not a recommendation to buy or sell any security. Any numbers above are illustrative, not live prices, and nothing here predicts price direction or rates a stock. Consider your own circumstances and consult a SEBI-registered investment adviser before acting.