A pizza layout is a model railway laid out as a circle of the smallest workable radius of curve, on the smallest possible square or circular baseboard. This baseboard can be so small as to look as if it would fit into a pizza box, hence the name. Building this minimum-size layout requires a small gauge. Most pizzas use 9mm gauge, as for N gauge. Z gauge and the esoteric smaller gauges would be useful too, but their costs are more than 9mm and there are fewer options available. Other gauges up to the popular H0/OO are also used, but almost only as narrow gauge models.