The best shapes for ragu bolognese
Finely minced beef and pork cooked with soffritto, milk and wine, and only a small amount of tomato, simmered for hours until the fat and the meat stop being separate things. It is far paler and far less tomatoey than most people outside Italy expect.
Finely ground, fat-rich, barely tomatoed. The fat wants a porous egg ribbon, the solids want folds to hide in. Bologna serves it on tagliatelle, never spaghetti.
Canonical: tagliatelle, lasagna.

The dough is rich and the filling soft, milky ricotta against salty cured mortadella and nutmeg, so the dressing stays light to let the parcel speak.

A cannelloni does not catch sauce, it contains the meal.

A fazzoletto is a wide, very thin sheet, so it presents a large flat face to its dressing rather than a groove or a twist to trap it.

The shape works two ways at once.

A lasagna sheet is less a noodle than a building material.

This is a very fine, porous all-egg ribbon, so it drinks up a light meat sauce rather than carrying a heavy one.

Width is the whole argument.

A wide flat ribbon gives sauce more landing area per strand than any round noodle.
What we would not put it on
Spaghetti. This is the best known mismatch in Italian cooking: a round smooth strand cannot hold a fat-rich mince, and Bologna serves the sauce on a porous egg ribbon instead.
Want it the other way around? Start from a shape in the Pairing Finder.
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