The completeness of the language material collected by the comparative linguistics makes possible the further step in the development of the historical syntax which is to place the comparative studies into the system of views formed lately in the linguistics. The modern historical syntax is to begin with the classification of languages as oral and written. Some important language characteristics depend on the type of the text. In the first instance it is about the main syntactical unit. An oral text presupposes that the main syntactic unit is a period, which is determined on the base of both structural and rhythmical characteristics. The latter depend on the situation of the text performance and are bound with memory, breath, etc. The main unit of a written text is a phrase, determined only structurally. But whatever the main unit of syntax would be: a period or a phrase, its basic characteristic is the same. This is the predicativity. It is necessary to distinguish the predicativity as an ontological phrase characteristic from its technical realization. Ontologically the predicativity is the connection between linguistic structures and the outer world. Technically the predicativity represents a kind of structure. The completeness of the structure depends on the self-sufficiency of the outer factors to make the connection between linguistic structures and the outer world obvious. Mononuclear phrases provide the highest degree of obviousness. Whereas such a connection is unevident it is to be guaranteed by the structure. In this case the structure of the main syntactical unit has to be more complicated. It represents a dependence tree with its own head. The type of the head depends on the main syntactical unit type. For a phrase it is a verb. For a period it is a noun.