Although both are Italian, tagliatelle and pappardelle are different and are not traditionally substituted for each other in the kitchen by Italians. However, there's no reason these two kinds of pasta can't be substituted in recipes, and many non-Italians do so and are more than happy with the results.

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Tagliatelle vs. Pappardelle
Tagliatelle and pappardelle come from a pasta category known as "the cutters," and both are made from egg dough.
They're both flat, long, and wide; however, tagliatelle looks like a long, flat ribbon and is 6.5 to 10 mm wide, whereas pappardelle is about 2 to 3 cm wide.

More About Tagliatelle
Tagliatelle is popular worldwide but originated in Bologna, in the Emilia-Romagna region of Northern Italy. Tagliatelle translates as "to cut," and it's very similar to fettucine.
It is best served with rich meat gravies, ragu, mushroom sauces, or pesto, but it's delicious with any sauce.
How To Prepare Tagliatelle
Dried tagliatelle should be boiled in salted water for seven to ten minutes. Fresh should only take three to four minutes.
More About Pappardelle
Pappardelle comes from the verb "to gobble up" in Italian, which is exactly what people do with this pasta. It originated in Tuscany in central Italy in the 14th century, and it's now found all over Italy.
Pappardelle is usually served in rich sauces, such as Bolognese or mushroom sauce, and it's particularly popular in the winter.
How to Prepare Pappardelle
Dried pappardelle should be boiled in salted water for eight to ten minutes. Fresh pappardelle needs three to four minutes.
How Many Types of Pasta Are There?
There are at least 350 different types and an estimated 1,400 names for pasta.
Can Tagliatelle and Pappardelle Substitute for Each Other?
Italians believe you cannot, but many people use tagliatelle and pappardelle interchangeably. Ideally, you'll use the one the recipe calls for, but both are delicious with any sauce.
How To Make Tagliatelle and Pappardelle Noodles from Scratch
Both are made using the same recipe; all that differs is the cut. Mix flour, eggs, salt, water, and olive oil into a dough, then knead it for at least ten minutes.
Then, roll out the dough and use a pasta cutter, knife, or pizza cutter to make your pasta.