Finally, certain post-syntactic operations can affect morpheme order. The best studied one is Morphological Merger or Merger Under Adjacency. This operation merges two adjacent terminal nodes into one morphological word. In other words, it allows for two heads which are adjacent to merge into one word without syntactic head movement – the operation is post-syntactic. This operation is doing the work of, say, affix lowering of the past tense morpheme in English in early generative syntax. For the operation to apply, what is crucial is that the morphemes to be merged are linearly adjacent.