The difference between Fall in line and Knuckle under
When used as verbs, fall in line means to submit to the rules of a higher authority, whereas knuckle under means to yield or cooperate when pressured or forced to do so.
check bellow for the other definitions of Fall in line and Knuckle under
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Fall in line as a verb (idiomatic):
To submit to the rules of a higher authority; obey; conform.
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Knuckle under as a verb (intransitive, idiomatic):
To yield or cooperate when pressured or forced to do so.