One of the objects of deconstruction is to undermine hierarchical structured binary oppositions within a text. This implies that a deconstructive strategy pays special attention to words or concepts that cannot be adopted into such a binary logic. They may be termed as undecidables, unsettled concepts. These words or concepts need to be elicited from the text that is being deconstructed (that deconstructs itself). If they would remain exterior to the text, the text would remain untouched. This means that such undecidables are not universal concepts: in each text, new and other undecidables will present themselves.
Undecidables are characterized by their virtue of being able to function within certain oppositions that are essential for a certain argumentation, but undermine these oppositions at the same time because of their double meaning. Derrida describes undecidables as verbal properties that can no longer be included within philosophical (binary) oppositions; they resist and disorganize such oppositions without ever constituting a third term and without ever leaving room for a solution in the form of speculative dialectics, a Hegelian Aufhebung (cf. Positions, p.43). Undecidables mark the points of what can never be mediated, mastered, or dialecticized. A few examples: pharmakon (this means both poison and remedy), supplement (addition and replacement), difference (distinction and deferral), communication (oral presentation and transmission of messages). Undecidables graft one meaning onto another; they take up a key role as they bring together and separate possible meanings at the same time. Their meaning cannot be presented as 'this and that' or 'this or that'. It is 'and' and 'or' at the same time. The pharmakon has no proper or determinate character; rather, it is the possibility of both poison and remedy. It is ambivalent because it constitutes the element in which opposites are opposed, the movement and play by which each meaning relates back to the other. Undecidables are the movement, the locus, the play of difference. An operation that at once brings about a fusion or confusion between opposites, and stands between opposites. A double and impossible operation (cf. Culler, p.145). According to Derrida, words of this type '... situate perhaps better than others the places where discourses can no longer dominate, judge, decide: between the positive and the negative, the good and the bad, the true and the false' (Points, p.86).
 Translations in particular often fail to accurately reproduce the ambiguousness of an undecidable in its flowing and floating meaning. By attempting to assign an unambiguousness meaning to an undecidable, its surprising dynamics becomes lost. By interrupting the transition of the opposite meaning, the moving and playful web of a text becomes neutralized.
Derrida inscribes his own texts on the places of these undecidables in order to demonstrate or provoke their differentiating function. But he goes on to say that the undecidability is not really due to the various meanings of pharmakon, supplement, difference, etc; they can only produce their undecided effects through syntax, the placement and grammatical status of a word in a sentence. The shift of the meaning of a word, this back and forth shift in the syntax causes the non-binary logic: neither/nor, that is simultaneously either/or. It is the formal and syntactic praxis that composes and decomposes the lexical richness and semantic openness of a word or concept. There is an irreducible excess of the syntactic over the semantic (cf. Dissemination, p.220).
 Undecidability cannot be confused with simple indecision, a pure paralyzation by the play of signifiers. On the contrary, undecidability is the condition of a possibility of acting and deciding; it is a determinate oscillation between possibilities that are themselves highly (pragmatically) determined in strictly defined situations. There would be no undecidability if it were not between determined poles. But whenever a decision is really a decision, whenever it is more than programmability or calculability, it is because it has passed through the ordeal of undecidability; it depends upon undecidability, which gives us something to decide. Deciding is thus a possibility sustained by its impossibility. The undecidability is never set aside, never over and done with (cf. Limited Inc., p.116 and p.148).
 Thinking in oppositions means that each of the opposed terms must simply be external to the other. The opposition between the inside and the outside can thus be seen as the matrix of all possible oppositions. However, undecidables have no proper place within such a frame of thought. They indicate the impossibility of a definitive separation between inside and outside. (Consider, for example, the quotations with which some of these pages open. They cease to be quotations, pinned to the outside of the text by position and material form, from the moment they work inside the very body of the text.) On one hand, it is the undecidables that enable or activate the opposition; on the other hand, they are themselves capable of escaping it. Thinking in opposites is taken to a limit at which point a certain displacement of the series of oppositions takes place. 'We cannot qualify it, name it, comprehend it under a simple concept without immediately being off the mark' (Dissemination, p.104).
 On the page entitled The Pharmakon, an example of an undecidable that occurs in Derrida's text 'Plato's Pharmacy' (Dissemination, p. 63-171) is elaborated upon. Examples of musical undecidables can be found on the pages Of New Musicology, Music and/as Disorder and Hymen.