Italian tube pasta come in as many sizes as plumbing pipes. Tubetti fit in roughly between ziti and ditali, and tubettini, as you might guess, are smaller.
Tubetti appear in minestras, with peas or cheese (cacio, usually) or with various sauces. Down in Puglia, they like them with mussels (alle cozze) or monkfish (alla rana pescatrice) or other seafood.
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