An international agreement is often reported as concluded on the day it is signed. Signature is an early step, and the legal obligations usually begin much later, if they begin at all.
Signature and consent are different acts
Signing a treaty generally indicates that a negotiating state accepts the text as final and intends to consider being bound by it.
Consent to be bound comes through ratification, acceptance or accession, each of which is a separate deposit of an instrument with a designated depositary.
Between those two acts a state is expected not to defeat the treaty's object and purpose, but it is not yet obliged to perform what the treaty requires.
Domestic approval sits in the middle
Ratification is a national process, and its shape depends on each country's constitution. In the United States, the Senate provides advice and consent for treaties before the president ratifies.
Legislatures often need implementing laws as well, because a treaty commitment that requires new agency powers or funding cannot be met without domestic authority.
That work can outlast an administration. A text negotiated by one government may be ratified by another, or left pending indefinitely without formal rejection.
Entry into force waits for a threshold
Most multilateral treaties specify how many ratifications are required before they take effect, and sometimes which kinds of states must be among them.
Thresholds are designed so that an agreement does not begin operating without enough participants to make it meaningful, particularly where obligations are reciprocal.
Once the threshold is met, the treaty enters into force for the states that have ratified. Others join later, and it enters into force for each of them individually.
Reservations narrow what a state accepts
A state may ratify while excluding or modifying particular provisions, provided the treaty permits it and the reservation is compatible with the agreement's purpose.
Other parties can object, which produces a patchwork in which the same treaty imposes slightly different obligations between different pairs of states.
Understanding what a country has actually agreed to therefore requires reading its instrument of ratification, not only the treaty text.
Provisional application fills the gap
Some agreements allow parts to be applied provisionally before entry into force, so that arrangements requiring immediate operation are not stalled by ratification timetables.
This is common where an agreement replaces an expiring one, or where administrative bodies must exist before the substantive rules can function.
Provisional application is a practical device rather than a shortcut, and it can be withdrawn, which is why it is negotiated as carefully as the treaty itself.