Naan resembles pita bread and, like pita bread, is usually leavened with yeast; unleavened dough (similar to that used for roti) is also used. Naan is cooked in a tandoor, or clay oven, from which tandoori cooking takes its name. This distinguishes it from roti which is usually cooked on a flat or slightly concave iron griddle called a tava. Modern recipes sometimes substitute baking powder for the yeast. Milk or yoghurt may also be used to give greater volume and thickness to the naan.