Economists talk loosely about substitutes and complements as if each pair of goods can always be characterized as one or the other.
That’s incorrect: their substitutability can depend on the situation, particularly the time and the individual’s circumstances, even for the same person. An acquaintance of mine reported the perhaps-apocryphal story that a major discount store is offering any customer who buys diapers in three different sizes a free package of condoms.
The managers thus assume that for such people, diapers and condoms are complements. Obviously that could not have been true when the event that created the eventual demand curve for diapers occurred!
I wonder, what are other examples of goods that can switch between being complements and substitutes depending on circumstances?