This paper addresses the following question: What kind of properties must the structure-building operation Merge have such that, given a Numeration, the grammar will build the ‘right’ structure and avoid generating ill-formed configurations? The answer we will propose is that Merge should be seen as an asymmetric operation in the sense of relating two items whose sets of morpho-syntactic features are in a proper inclusion relation. In addition, we propose a partition of features into two stacks: categorial features and operator features. This distinction is independently motivated as it feeds into the definition of External Merge and Internal Merge (Chomsky’s 2001). The proper inclusion condition will be assumed to hold for both of these operations, but the set of features under consideration for the evaluation of the proper inclusion relation differs: strictly categorial features for External Merge, and the whole set of features of lexical items for Internal Merge.