"Each Other" & "One Another"

This is not a concept that will be tested on the GMAT.

To my ears (as a native English speaker, literature major, and professional writer/educator/editor), these two phrases are interchangeable:
Daenerys and Jon Snow looked at each other.
Daenerys and Jon Snow looked at one another.
These both seem perfectly acceptable to me.

But of course, we can't simply trust our ear on the GMAT! And the GMAT (like any grammar source) will be particular about some idioms but not others. So I looked it up, and it turns out that there is plenty of disagreement on the issue:
https://www.quickanddirtytips.com/educat ... ne-another
https://learnenglish.britishcouncil.org ... ne-another
https://grammarist.com/usage/each-other-one-another/

I can confirm, however, that this is not a rule I've seen tested on the GMAT. A search of all the SC questions in OG 2017 revealed only 2 questions that contained either phrase:

SC #747:
In California, a lack of genetic variation in the Argentine ant has allowed the species to spread widely; due to their being so genetically similar to one another, the ants consider all their fellows to be a close relative and thus do not engage in the kind of fierce intercolony struggles that limits the spread of this species in its native Argentina.

due to their being so genetically similar to one another, the ants consider all their fellows to be a close relative and thus do not engage in the kind of fierce intercolony struggles that limits
due to its being so genetically similar, the ant considers all its fellows to be a close relative and thus does not engage in the kind of fierce intercolony struggles that limit
because it is so genetically similar, the ant considers all its fellows to be close relatives and thus does not engage in the kind of fierce intercolony struggles that limits
because they are so genetically similar to one another, the ants consider all their fellows to be close relatives and thus do not engage in the kind of fierce intercolony struggles that limit
because of being so genetically similar to one another, the ants consider all their fellows to be a close relative and thus do not engage in the kind of fierce intercolony struggles that limits

SC #797:
Almost like clones in their similarity to one another, the cheetah species' homogeneity makes them especially vulnerable to disease.
the cheetah species' homogeneity makes them especially vulnerable to disease
the cheetah species is especially vulnerable to disease because of its homogeneity
the homogeneity of the cheetah species makes it especially vulnerable to disease
homogeneity makes members of the cheetah species especially vulnerable to disease
members of the cheetah species are especially vulnerable to disease because of their homogeneity

None of the answer choices required us to choose between "each other" and "one another." I cannot think of any example from any older OG that does ask us to make this choice.

This is not an issue you should worry about.

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