Pair of concepts
couple of terms whose meanings depend on each other
art and nature
synchrony and diachrony
tangent and cotangent
sine and cosine
secant and cosecant
positive and negative numbers
signified and signifier
vices and virtues
takeoff and landing
precision and recall
binary opposition
workweek and weekend
letter and spirit of the law
literal and figurative language
necessity and sufficiency
creation and annihilation operators