Post-Only Instruction

Action

A Post-Only Instruction, within electronic trading systems, mandates that an order can only be submitted as a ‘maker’ order, actively adding liquidity to the order book. This directive prevents aggressive ‘taker’ orders that immediately execute against existing liquidity, potentially impacting price discovery. Exchanges implement this to encourage liquidity provision and reduce adverse selection, particularly in high-frequency trading environments. Consequently, the instruction is frequently utilized by algorithmic traders and market makers seeking to minimize execution costs and maintain a passive trading profile.