Periodic call auction is a method of trading used when it is greatly necessary to set an equilibrium price through the concentration of demand and supply, and it receives offers and bids for a certain time and then trading is concluded in a single price in accordance with the principle of price and time priority.
Every periodic call auction is closed at the random time that the KRX decides within 30 seconds from the closing point in order to prevent any unfair trading practices. With a random time element to the auction, market participants can no longer be certain of the exact time that the stock moves into a new state (or period), such as auction match, or an auction extension.
The priority order of offer price, participating in periodic call auction, was considered in the past as simultaneous offer price and the principle of time priority was not applied to, but the principle of time priority has been applied to in principle since September 2001 with the exceptional allowance of simultaneous offer price.
Accordingly, as for offer prices which take part in periodic call auction, trading of whole quantities between offer prices with priority shall be concluded in compliance with the principle of price and time priority, and no allocation of quantities shall be made between those of same offer prices. However, as far as opening price is formed in the upper or lower limit price, allocation of concluded quantities is made to those at buying offer price of the corresponding price range in the case of the upper price limit while such allocation is made to those at selling offer price of the corresponding price range in the case of lower limit price. This is to satisfy minimal demand of investors who want to conclude trading within the day by way of allotting quantities if opening price is formed in the upper or lower limit price.
As shown in the following example, trading is made in such way that the highest offer price for buying and the lower offer price for selling are concluded in regular sequence, and the principle of time priority is also applied to the same price of KRW15,250 for trading to be concluded.
| Selling quantity | Price | Buying quantity |
|---|---|---|
| 15,400 | ● 1000 | |
| ○ ○ | 15,350 | ● 300 |
| ○ ○ | 15,300 | ● 200 |
| ○ ○ ◑ ● 2,000 1,000 500 100 |
15,250 | ● 200 ● 300 |
| 150 ● | 15,200 | ○ ○ |
| 150 ● 500 ● | 15,150 | ○ |
| 500 ● | 15,100 | ○ ○ ○ |
| 150 ● | 15,050 |