It's baked in long rectangular trays and sold by weight, not by the slice, so a Roman order is literally however many grams you point at and ask for.
Pizza al taglio is baked in large rectangular trays with a thicker, airier crust than Neapolitan pizza, topped with anything from simple olive oil and salt (pizza bianca) to potatoes or vegetables, then cut with scissors and sold by weight. Roman bakeries and pizzerie keep trays going all day for a constant stream of walk-up customers.
Arancini (Fried Rice Balls)
Lampredotto
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Campari