In the present paper we discuss some issues connected with the condition of projectivity in a dependency based description of language (see Sgall, Hajičová, and Panevová (1986), Hajičová, Partee, and Sgall (1998)), with a special regard to the annotation scheme of the Prague Dependency Treebank (PDT, see Hajič (1998)). After a short Introduction (Section 1), the condition of projectivity is discussed in more detail in Section 2, presenting its formal definition and formulating an algorithm for testing this condition on a subtree (Section 2.1); the introduction of the condition of projectivity in a formal description of language is briefly substantiated in Section 2.2. and some problematic cases are discussed in Section 2.3. In Section 3, a preliminary classification into three main groups and several subgroups of Czech non-projective constructions on the analytical level is presented (Section 3.1),