What is an essential characteristic of accessory cord when used for prusiks?

The essential characteristic of accessory cord when used for prusiks is that it should be 2/3 to 3/4 the diameter of the main rope. This sizing ensures that the prusik knot can grip the main rope effectively while allowing for ease of adjustment. When the diameter of the accessory cord is appropriately sized relative to the main rope, it creates enough friction to hold securely under load, enabling the user to ascend or descend safely.

Using an accessory cord that is too thick would prevent the prusik from gripping the main rope properly, potentially compromising safety during operations. Conversely, if the accessory cord is too thin, it may not provide sufficient friction or could easily slip. Therefore, the diameter ratio is critical for the effectiveness and safety of the prusik knot in rope rescue scenarios.