The Periodic Call Auction (PCA) mechanism was introduced by the Securities and Exchange Board of India (SEBI) in 2013. This initiative was designed to bring stability and fair trading to illiquid stocks by implementing structured trading sessions throughout the trading day.

According to SEBI, illiquid stocks are defined as follows:

1. Shares with an average daily number of trades of fewer than 50
2. Shares with a daily trading volume of less than 10,000, among other similar conditions.

Under the PCA mechanism, there are six call auction sessions throughout the trading day, each lasting for one hour. These sessions begin at 9:30 AM along with the regular trading session. This auction window is structured similarly to the pre-market session and follows this pattern:

1. 45 minutes for order placement and modification by investors.
2. 8 minutes for order matching and confirmation by the Exchanges.
3. 7 minutes of buffer time before the next session.

Session No	Start Time- Order Placement	Order matching	Buffer period
1	09:30 AM - 10:15 AM	10:15 AM - 10:23 AM	10:24 AM to 10:30 AM
2	10:30 AM - 11:15 AM	11:15 AM - 11:23 AM	11:24 AM to 11:30 AM
3	11:30 AM - 12:15 PM	12:15 PM - 12:23 PM	12:24 PM to 12:30 PM
4	12:30 PM - 01:15 PM	01:15 PM - 01:23 PM	01:24 PM to 01:30 PM
5	01:30 PM - 02:15 PM	02:15 PM - 02:23 PM	02:24 PM to 02:30 PM
6	02:30 PM - 03:15 PM	03:15 PM - 03:23 PM	03:24 PM to 03:30 PM

If you intend to trade illiquid stocks included in the periodic call auction, orders for these Stocks must be placed within the initial 45-minute period of the call auction session. You can buy or sell depending on whether the order matches within the following 8 minutes.

For a list of Stocks that are under periodic call auction, please refer NSE & BSE.

To learn more about the periodic call auction, you can refer to the BSE FAQs and NSE Circular.

Note: If an order is placed but not executed during a session, it will be automatically reattempted for execution in the subsequent sessions throughout the trading day.