noun
Typically used for mated animals or a matched pair of things; often seen in contexts like bird pairs or a pair of wings.
池には鴨のつがいが泳いでいた。
A pair of ducks was swimming in the pond.
この鳥は一生同じつがいで過ごす。
This bird stays with the same mate for life.
noun
Anatomical term for a joint, such as a hinge joint in arthropods or a suture in a skull; not a common everyday word.
昆虫の脚のつがいの構造を観察した。
I observed the structure of the insect leg joint.
ペア is a general loanword for any pair, while つがい often implies a natural or mated pair, especially of animals.
夫婦 refers specifically to a married human couple, whereas つがい is used for animals or inanimate matched pairs.
関節 is the standard anatomical term for joints in vertebrates; つがい is more specific to certain biological contexts like arthropod joints.
Derived from the verb 番う (つがう, 'to pair, to join'). The anatomical sense likely developed from the idea of two parts fitting together.