Hard, salty, sharp sheep's-milk cheese aged at least five months — the cheese of cacio e pepe, carbonara, gricia and amatriciana, though most of it is now made in Sardinia.
Italian delis, in a piece. Grate it yourself on a fine microplane; pre-grated clumps.
Worth knowing
It's much saltier than parmesan, so go light on salt in the pasta water. For cacio e pepe that doesn't seize, beat the finely grated cheese with a little cooled pasta water into a paste first, then loosen it into the pasta off the heat.