In Italy, ragù alla bolognese is traditionally served with tagliatelle rather than spaghetti.
The classic Bolognese pairing is tagliatelle al ragù: fresh, broad, flat egg pasta from the Emilia-Romagna region. Its width and texture help the slowly cooked meat sauce cling to the noodles. Ragù alla bolognese is generally made with minced or finely chopped meat, a soffritto of onion, celery, and carrot, wine, a little tomato, and often milk, then simmered gently for a long time.
Spaghetti bolognese is extremely popular outside Italy, especially in English-speaking countries, but it is not the traditional Bolognese combination. The international dish may reflect immigrant cooking, adaptations for exported dried pasta, or the influence of tomato-rich southern Italian sauces.
Pappardelle and fettuccine can also carry a meat ragù effectively, and rigatoni or penne may be used in some settings. However, the single canonical answer for the traditional Bologna pairing is tagliatelle. The important distinction is between a fair substitute and the named local tradition.