The paper addresses the question of the correspondence between constituent order in compounds and in syntax. While a strictly synchronic perspective does not lead us to any significant generalization as ascertained by Bauer (Language typology and language universals, Mouton de Gruyter, Berlin 2001), adopting a diachronic point of view allows us to formulate the question in general terms by making reference to the logical problem of what is the transition permitted from a certain synchronic stage to another. On the basis of a large language sample, it is shown that constituent order in compounds heavily relies on syntax. This must be understood in the terms of a diachronic universal reflecting Hawkins' (Word order universals, Academic Press, New York 1983) Double Acquisition Hypothesis. For this specific property of compounds, morphology does not seem to be autonomous from syntax, albeit the relation between morphology and syntax must be thought of as a multi-faceted one.