 Neither question (1) nor question (2) will be addressed here. Instead, this entry focusses on the properties of—and relations between—the formal representatives of these connectives. Examples of such properties include truth-functionality and a replacement property called congruentiality, and of such relations what we call the subconnective relation—relating one connective to another when (roughly speaking) every logical principle holding for the first of a pair of connectives also holds for the second. In fact, all of these concepts are subject to a further relativity. For instance a connective is or is not congruential according to a given logic, with a similar relativity in the case of whether one connective is a subconnective of another. On the other hand, as truth-functionality is defined below, a connective is or is not truth-functional with respect to a class of truth-value assignments—and various interesting further relations give rise to a corresponding logic-relative version of this concept depending on the different ways a class of truth-value assignments is linked to a logic (which in turn depends on how we think of a logic—as an ordinary consequence relation, for example, or as a ‘multiple conclusion’ consequence relation: see further note 1 below). For some other properties, the further relativity is a bit different. For example, whether a given connective is uniquely characterized—in the sense of being governed by rules which when reduplicated to govern a new connective render compounds formed from the same component sentences using the original and the new connective completely interchangeable—is relative not just to a logic but to a particular proof system (set of rules systematizing the logic).