For an international treaty to be legally binding upon a state it must be signed by the head of that state or government. In a letter sent by the head of state of a particular EU member, he/she informs that the state in question has ratified an EU Treaty.
The letter of ratification is sent to the Italian Government for historical reasons - the first treaties were signed in Rome.
The deposit of the letter or "instrument of ratification" is the final stage of the Treaty ratification process.
Once all letters of ratification has been sent there is no turning back. EU treaties come into force on an agreed timescale once letters (instruments) of ratification have been deposited by all member states.
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See also Ratification.