NATO progressive expansion to the east has generated prolific academic literature. However, only a few works have investigated how these enlargements affect the internal dynamics of the Alliance. This article thus seeks to analyse changes at work within NATO by focusing on informal groupings created by central and eastern European countries to influence its decision-making processes. Based on extensive fieldwork within the organisation, this paper will mostly focus on Bucharest nine, which seems to be the preferred extra and intra-alliance format. It will first analyse the custom of informal groupings within NATO and its main features. Then, this article will trace the emergence of Bucharest nine and its claims, to end with an assessment of its homogeneity.