 In TS, §§ 94–108, Bolzano defines a system of relations between the extensions of ideas, and proves various theorems about these relations. To begin with, ideas \(A, B, C, \ldots\) are said to be compatible just in case there is at least one object that stands under all of them; otherwise, they are called incompatible. In the case in which not only some but all objects represented by \(A\) are also represented by \(B\), \(A\) is said to be included in \(B\). If the converse also holds, i.e., if \(A\) is included in \(B\) and \(B\) included in \(A\), the ideas \(A\) and \(B\) are called equivalent (or interchangeable). He defines two further special cases: first, compatibility where neither \(A\) is included in \(B\), nor \(B\) in \(A\); Bolzano calls this relation overlapping or linking. Second, we have the relation of subordination, which is inclusion without reciprocity.