The term ‘word order’ stems from traditional occidental grammar, which was based on the grammar of Classical Latin. In this language, the order of words in a clause is relatively free. With some exceptions including the preposition and its complement, the components of a construction may generally be inverted or distanced.

Since the middle of the 20th cent., English grammar became prominent in linguistics and its properties have often been taken as the tertium comparationis in language comparison. Now in English, syntagmas generally form phrases whose components have fixed sequential order, and it is only at the level of the clause that its main components may shift their order. Consequently, the term ‘word order’ was largely abolished and replaced by ‘constituent order’.

As a matter of fact, there are at least two parameters of variation at work here:

1. To the extent that the concept of word can be applied in a consistent way to different languages, it is indeed the case that word order is more variable in some languages than in other languages. For instance, Latin word order is relatively free while English word order is relatively fixed. This is related to the richness of inflection of the word: A word which signals its syntactic function by its inflection does not need to signal it by its syntagmatic position, so this is available for information structure.
2. The levels of grammatical structure are in a hierarchy of complexity shown in the following diagram:

   Hierarchy of complexity levels of linguistic structure

   level	unit	linguistic operations
   discourse	text	rhetorical operations
   syntax	complex clause	syntactic operations
   	simple clause
   	syntagma
   morphology	word form	inflectional operations
   	stem	operations of stem formation
   	morpheme/root	[none]

   As a matter of universal principle, linguistic operations at higher levels of this hierarchy enjoy more freedom than operations at lower levels. One consequence of this is that a language may have free main constituent order but fixed word order inside lower level syntagmas, while the converse is excluded.

The first one to observe this latter parameter of variation in the order of meaningful elements was Joseph Greenberg:

On the whole, the higher the construction in an immediate constituent hierarchy, the freer the order of the constituent elements. It has been seen that practically all languages have some freedom of order regarding subject and predicate as a whole; whereas only a small minority have variant order in genitive constructions, and then almost always along with other differences, not merely a difference of order. Within morphological constructions, order is the most fixed of all.

(Greenberg 1963: 82)

Greenberg, Joseph H. 1963, „Some universals of grammar with particular reference to the order of meaningful elements.“ Greenberg, Joseph H. (ed.), Universals of language. Report of a conference held at Dobbs Ferry, New York, April 13-15, 1961. Cambridge, MA: MIT Press; 58-90.

This regularity was reframed by John Robert Ross in the immortal Penthouse Principle, which reads as follows:

More goes on upstairs than downstairs.

(Ross 1973:397)