The ribbon everything else is measured against
Tagliatelle come from Bologna, where the width is taken seriously enough that the local chamber of commerce once lodged a gold reference strip to settle arguments about it. Eight millimetres cooked is the Bolognese standard. Nobody in a Tuscan kitchen measures, but the target is the same.
Tuscany cooks them constantly regardless of where they were born, because they are the useful ribbon: wide enough for a substantial sauce, narrow enough to eat without a fight, and quick enough to be a Tuesday dinner rather than an occasion.
How they are cut
Roll the egg dough into a thin sheet, let it dry for a few minutes until it is leathery rather than tacky (this is the step people skip, and it is why their ribbons stick together), then flour it generously, roll it into a loose flat log and cut across with a sharp knife.
Then lift each cut immediately and shake it loose. Left in the roll, ribbons weld into a brick within minutes. Toss them in a little semolina and coil them into loose nests until the water boils. Tagliatelle are the shape that teaches you knife control, because at 6 mm a wandering cut is obvious in a way it never is on pappardelle.
What to put on them
A proper ragù (slow-cooked minced beef and pork with soffritto, wine and a very little tomato) is the pairing, and in Bologna it is the only one anybody will admit to. Note what it is not served with: spaghetti. That is a foreign invention and it will be pointed out to you.
Beyond ragù, tagliatelle are superb with mushrooms, with butter and sage when you want something quiet, and with a simple tomato and basil when the tomatoes are actually good. The ribbon is narrow enough that a light sauce still coats it properly.
Where to eat tagliatelle in Florence
Tagliatelle come from Emilia, but they are on almost every trattoria menu in Florence, most often al ragù or, from late summer into autumn, ai funghi porcini. Because they are so common, they are also the ribbon most often bought in rather than made in house, so this is the dish where reading the menu matters most.
My guide to handmade pasta in Florence explains how to tell the difference before you order. To take some back to your kitchen, here is where to buy fresh pasta in Florence and how long it keeps.