Spice rub is any mixture of groun spices that is made for the purpose of being rubbed on raw food before the food is cooked. The spice rub forms a coat on the food. The food can be marinated in the spice rub for some time for the flavors to incorporate into the food or it can be cooked immediately after it is coated in the rub. The spices are usually coarsely ground. In addition to spices, salt and sugar may be added to the rub, the salt for flavor and the sugar for caramelization. The simplest rub is just coarsely ground black pepper as in Steak au poivre.