A pair of glasses with dual number.
Dual number is a grammatical number equal to 2. It's marginally present in English as a vestige. Generally these are treated as a type of plural.
Examples include "scissors", "pants", "trousers", etc. A "pair of", or a "couple of" can be used as quantifiers.
English language learners may be reminded that they have two legs, so need a one trouser for each leg (and thus a pair of trousers). In other languages, the corresponding words may be singular.
In British English, this usually takes a plural verb agreement, e.g. "the couple are married", whereas in American English this tends to be "the couple is married". In this regard, the treatment is the same as collective nouns.
The indefinite pronouns/adverbs, "both", "either" and "neither" are dual.