Periodic call auctions are mechanisms introduced by exchanges to improve price discovery in illiquid stocks — those with very low trading volumes. These auctions occur at specific intervals during the day and are designed to ensure fair and transparent price formation.

Criteria for Illiquidity

A stock is considered illiquid and placed under periodic call auction if:

• It averages fewer than 10,000 shares traded daily
• It records fewer than 50 trades per day

Such stocks are not continuously traded during normal market hours but are instead grouped into scheduled call auction sessions.

How Does the Call Auction Work?

Each trading day, six call auction sessions are held for illiquid stocks. The structure of each session is as follows:

Order Lifecycle

• Order Placement Window: Traders can place, modify, or cancel orders.
• Order Matching Window: Buy and sell orders are matched based on price-time priority.
• Buffer Period: Used for transition and confirmation before the next session begins.

Example

Let’s say “ABC Ltd” is a rarely traded stock with fewer than 8,000 shares traded daily and about 30 trades. ABC Ltd would qualify for the periodic call auction mechanism. If you place an order during the auction window, it will be executed only if a matching counter-order exists in that session.

What If...

Note: Orders not executed in a session will roll over to the next. If they remain unexecuted by day’s end, they are cancelled and must be re-entered the next trading day.

Last updated: 28 Jun 2025