Ambassadors were only exchanged between great powers, close allies, and related monarchies.
[2] The envoy rank still existed in 1961, when the Vienna Convention on Diplomatic Relations was signed, but it did not outlive the decade.
The last remaining American legations, in the Warsaw Pact countries of Bulgaria and Hungary, were upgraded to embassies in 1966.
[4] The role of presbys, commonly translated as "envoy", was a kind of temporary, ad hoc ambassador with highly circumscribed powers, sent from one Greek polis to another to negotiate the resolution of a single, particular issue.
Moreover, the rank of envoy should not be confused with the position of Special Envoy, which is a relatively modern invention, appointed for a specific purpose rather than for bilateral diplomacy, and may be held by a person of any diplomatic rank or none (though usually held by an ambassador).