There are phrases which pair things up. For example, "checks and balances", "bells and whistles",
What is the rational behind this construct? Any more examples?
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One rationale for the pairings may just be that using them sounds better than a single word. They add a bit of rhythm to the sentence
It is a figure of style known since Antiquity, called hendiadys, "one through two": two parallel nominal words are used to express a single idea, which would ordinarily be expressed by a "head" word and an attribute. There is no reason for this phenomenon but style.
Off the top of my head, I would suggest two broad rationales for these idiomatic pairings: 1) contrast and 2) emphasis (based on the similarity between the words making up the pair).
checks and balances
day and night/night and day
do or die
dos and don'ts
give and take
[come] hell or high water
hill and dale
high and low
ins and outs
pros and cons
[come] rain or shine
ups and downs
airs and graces
an arm and a leg
be-all and end-all
bells and whistles
blood and guts/thunder
[rain] cats and dogs
cut and dried
day and age
dazed and desultory
[between] the devil and the deep blue sea
fire and brimstone
flesh and blood
high and mighty
Ps and Qs
plain and simple
[between] a rock and a hard place
simple and straightforward
There are some phrases that pair synonyms where one of the words is considered archaic, for example:
"Kith" and "kin" both mean "relatives" -- kith is archaic.
"Time" and "tide" both mean "time" -- tide, in this sense, is archaic (compare to "Zeit" and "tijd" in German and Dutch).
Cooper and Ross 1975 deal with both kinds of such English fixed conjoined phrases (which they call Freezes) -- the ones where the conjoined words have meaning, like
as well as the kind where they don't, like
They show that both types of freezes follow the the same phonological rules.