singular and plural; simplicity and complexity; singles and doubles
A compound noun that pairs opposite concepts: singular vs. plural (grammar), simple vs. complex, or singles vs. doubles (sports). The specific meaning depends on context.
In English, you need to pay attention to whether a noun is singular or plural.
It's difficult to determine whether this problem is simple or complex.
The tennis tournament is held in both singles and doubles.
same singular and plural form
Compound of 単 (simple, single) and 複 (complex, plural). The kanji directly convey the paired opposites.