Beef short ribs
Meaty bone-in pieces of beef rib, well marbled and rich in connective tissue.
Short ribs are sold in many supermarkets in English-speaking countries, less often in German-speaking ones, where the cut is called Querrippe. For Korean dishes it is worth asking a butcher: thinly cut across the bones for grilling, in thick pieces for braising. Asian grocers sometimes stock them already cut this way in the freezer.
Substitute:
For braising, beef cheek or oxtail work well and turn similarly succulent and gelatinous. For grilling, thinly sliced rib-eye or sirloin can be used, but without the bone and with less fat.