Article 23 Vienna Convention on the Law of Treaties

Procedure regarding reservations

1. A reservation, an express acceptance of a reservation and an objection to a reservation must be
   formulated in writing and communicated to the contracting States and other States entitled to become parties to the treaty.
2. If formulated when signing the treaty subject to ratification, acceptance or approval, a
   reservation must be formally confirmed by the reserving State when expressing its consent to be bound by the treaty. In such a case the reservation shall be considered as having been made on the date of its confirmation.
3. An express acceptance of, or an objection to, a reservation made previously to confirmation of
   the reservation does not itself require confirmation.
4. The withdrawal of a reservation or of an objection to a reservation must be formulated in writing.

See also: Article 22 Vienna Convention on the Law of Treaties