  Unless otherwise stated, page and (sub)section references in this initial paragraph all pertain to Humberstone (2011), where several of the topics touched on this entry are treated at greater length. For example, the treatment of unique characterization in Section  3 of the present entry is largely based on §4.3 there. (Some issues touched on here are, however, not treated there at all or are addressed in greater detail here.) In connection with note 6 in the present entry, concerning Carnap's observation that the most familiar logical frameworks do not force a Boolean interpretation on the connectives treated by the rules they make available, references to some of the many discussions of this theme can be found under ‘Strong vs. Weak Claim’ on p. 101. Apropos of note 10 (on sentence-position extensionality) in the present entry, referring the reader to this opening paragraph, see §3.2 and subsections 4.37–8. For the end of note 11 (the commutativity/symmetry distinction etc.), what is relevant is subsection 3.34, “Operations vs. Relations” (though for more on specifically the relational side of that contrast, see Humberstone 2013, especially §4). In connection with note 17 (on ‘non-homophonic’ semantic treatments of disjunction), see subsections 6.43–6.46 for a survey of some of these treatments, and for note 21 (on separately conservative but jointly non-conservative extension by new connectives with prescribed inferential properties), see p. 568f.  For note 32 (on some of Rautenberg's work on what we are calling hybrids of connectives), see alternatively subsection 3.24. For note 41 (appended to a discussion of contrariety and subcontrariety), a more detailed discussion can be found in subsection 8.11 and references given there (especially in Remark 8.11.1). Expanding on note 45: §5.2 goes into more detail on logical subtraction (Example 4.4 above); interesting work by Stephen Yablo on this topic, mentioned there as then unpublished, subsequently appeared in Yablo (2014), Chapters 8 and 9 being of particular relevance. Apropos of note 55: for an example of a Hasse diagram of the 16-element Boolean algebra, nodes labelled with representatives of the classical equivalence classes of two-variable formulas, see Figure 2.13a on p. 225. The bearing on logical theory of the topic of the following paragraph of the present section, Galois connections, is stressed in material on pp. 58f., 75 and 80 in §1.1; the list of ‘Galois connection’ oriented discussions of logical matters given in the bottom paragraph of p. 101 should be supplemented by a reference to Bimbó and Dunn (2008). Finally, to supplement the Urquhart reference given at the end of the present section on head linkage as a special relation in intuitionistic logic, see subsection 9.25. The remainder of the present section gives further additional notes and sources on the present entry working through in order.