Editing Distributed morphology (section)

===Morpheme order===

In Distributed Morphology, the linear order of morphemes is determined by their hierarchical position in the syntactic structure, as well as by certain post-syntactic operations. Head movement is the main syntactic operation determining morpheme order, while Morphological Merger (or Merger under Adjacency) is the main post-syntactic operation targeting affix order. Other post-syntactic operations that might affect morpheme order are Lowering and Local Dislocation (see previous section for details on these operations).

====Head movement====
Head movement is subject to the Head Movement Constraint, according to which when a head moves, it cannot skip an intervening head. Left adjunction and the Head Movement Constraint ensure that the Mirror Principle holds. The syntactic mechanism responsible for the effects of the Mirror Principle is head movement: heads raise and left-adjoin to higher heads. The general principle behind morpheme order is the Mirror Principle (first formulated by Baker 1985), according to which the linear order of morphemes is the mirror image of the hierarchy of syntactic projections. For example, in a plural noun like ''cat-s'', the plural morpheme is higher in the hierarchy than the noun: [<sub>NumP</sub> -s[<sub>NP</sub> cat]]. The Mirror Principle dictates that the linear order of the plural morpheme with respect to the noun should be the mirror image of their hierarchy, namely the attested ''cat-s''. 
Research subsequent to Baker (1985) has shown that there are some apparent violations to the Mirror Principle and that there are more operations involved in the determination of the final linear order of morphemes. Firstly, the left adjunction requirement of head movement has been relaxed, as right adjunction has been shown to be possible (Harley 2010 among others{{by whom|date=March 2019}}). Therefore, different heads can have a specification for right vs. left adjunction. We could imagine, for example, that there is a language in which head movement of the noun to the Number head is specified for right adjunction, rather than left adjunction which is the case in English. In that language, the predicted order of the noun and plural morpheme would then be ''s-cat''. Notice, however, that right vs. left adjunction determines whether an affix will be realized as a prefix or a suffix: its closeness to the root will still reflect hierarchical order. Let's look at a hypothetical example to make this clear. Assuming that Tense is merged higher than Aspect, there are four possible orders for the tense and aspect morphemes with respect to the verb stem, once we allow for variation between left and right adjunction in head movement.

# Verb – Aspect – Tense
# Tense – Verb – Aspect 
# Aspect-Verb-Tense
# Tense-Aspect-Verb

Crucially, though, the following two orders are predicted to be impossible.

# Aspect-Tense- Verb 
# Verb – Tense – Aspect

These orders are the orders in which the tense morpheme is closer to the root than the aspect morpheme. Since Aspect is merged before Tense and morpheme order still reflects hierarchical order, such a configuration is predicted to be impossible.

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.