Det fremgik allerede af artikel 28 i konventionen af 1957, at de kontraherende parter kunne indgå bilaterale eller multilaterale overenskomster indbyrdes for at supplere denne konventions bestemmelser eller for at lette anvendelsen af de deri indeholdte principper.
25Simi-larly, the 1957 Convention already provided, in Article 28, for the contracting parties to be able to conclude bilateral or multilateral agreements between themselves to supplement its provisions or facilitate the application of its principles.