Siamese twins
Definition -
(1) A group of words that is always used together as an idiomatic expression or collocation.
Note: The order is important. If it is reversed, then the twins lose their idiomatic meaning. For example, we always say back and forth, never forth and back.
(2) Pairs of words linked by and or or that are used to convey a single meaning.

Example -
hue and cry, kith and kin, might and main, and rack and ruin

Etymology -
H.W. Fowler in Modern English Usage coined the term so that he would have an entry heading under which he could describe this phenomenon.

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