What's the numbering system for spaghetti and does it matter?

Question

When I lived in Italy some years ago I remember an Italian friend explaining the numbering system for spaghetti (perhaps also other long pasta). How spaghetti was sold in Italy with a number indicating its fineness.

She told me that different kinds of recipes called for specific numbered spaghetti, if you wanted to be precise.

Can anyone else explain the numbering system for spaghetti better than my vague recollection? Do people in Italy really worry about matching the right numbered spaghetti with a given dish, and has anyone outside Italy encountered a recipe calling for a specific number spaghetti?

Answer

They are just a "product number", and it may vary for the same kind of pasta from a manufacturer to another.

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