dyad

dyadi A set of two elements treated as one; a pair.
{{quote-book|year=1908|author=W. D. Ross|title=S:Metaphysics/Book I|Metaphysics|by=Aristotle
puhekieltä any set of two different pitch classes.
A pair of things standing in particular relation; dyadic relation.
"For each individual in a specific dyad (i.e., mother-offspring, offspring-father, sibling-sibling),..." Debra Lieberman, John Tooby, and Leda Cosmides - The evolution of human incest avoidance mechanisms: an evolutionary psychological approach, p. 20
puhekieltä An element, atom, or radical having a valence or combining power of two.