Best Price Guarantee
Best price guarantee in the context of exchange-traded markets refers to the price-time priority matching algorithm used by NSE and BSE, which ensures every incoming order is executed at the best available price in the order book before moving to less favourable prices.
The term 'best price guarantee' is more commonly used in retail brokerage marketing, but its operational basis in Indian exchanges is the price-time priority (PTP) matching algorithm — also called strict price-time priority or FIFO (first-in, first-out) within the same price level — that governs how the electronic limit order book matches orders.
Under price-time priority, every incoming aggressive order (a market order or a limit order priced at or through the best available opposite-side quote) is matched against the resting order with the best price first. Among all resting orders at the same price level, the order that arrived earliest in time has execution priority. This means a buy order submitted at the market will be filled at the lowest available ask price first; if multiple sellers are offering at that best price, the one who posted the order earliest gets filled first.
The practical implication for retail traders on NSE is significant: all participants — retail, institutional, algorithmic — compete for execution priority on identical terms within the same price level. Unlike over-the-counter (OTC) markets such as the currency forward market or the bond market, where a dealer might offer different prices to different counterparties based on relationship, the exchange order book is transparent and non-discriminatory.
However, co-location services — where market makers and HFT firms place their servers physically inside NSE's data centre — create a de facto speed advantage that allows these participants to post and modify orders microseconds faster than geographically remote traders. This does not violate price-time priority rules because co-location access is commercially available to any SEBI-registered entity, but it means that a retail trader with a standard internet connection will almost always lose a time-priority race at the same price level against a co-located algorithm.
SEBI has periodically reviewed co-location access policies at NSE following the 2015–2016 co-location controversy, which alleged that certain brokers received preferential early access to market data feeds. The resulting SEBI order and Supreme Court proceedings reinforced that all co-location clients must receive data simultaneously, preserving the theoretical fairness of price-time priority even in a high-frequency environment.