문법책들을 보면 간혹 each other는 둘 사이, one another는 셋 이상 사이에 쓴다는 설명이 나오고, 수에 따라 이를 구분하는 연습문제를 실어놓곤 한다. 하지만 현대영어에서 이 둘은 숫자에 관계없이 같은 의미로 쓰인다. 

1. Merriam Webster's Dictionary of English Usage를 보면 이러한 규칙의 유래부터, 이것이 왜 엉터리인지까지 매우 자세하게 설명되어 있다.

Bardeen 1883 indicates that the use of each other for one another is legitimate, though carped at by some critics; Bardeen had found his carping critic in Ayres 1881. Actually the prescriptive rule that each other is to be restricted to two and one another to more than two goes back even farther than Ayres. Goold Brown 1851 cites the rule with approbation, and quotes it from an even earlier grammarian, one T. O. Churchill (A New Grammar of the English Language, London, 1823). Churchill did not invent the rule; although Brown mentions no earlier instance, Sundby et al. 1991 have found it in a 1785 grammar by a Georgy N. Ussher. But Goold Brown also notes that “misapplications of the forgoing reciprocal terms are very frequent in books” and goes on to cite Samuel Johnson and Noah Webster in error. He further notes that “it is strange that phrases so very common should not be rightly understood.” It is perhaps easier now than it was in 1851 to see why: evidence in the OED shows that the restriction has never existed in practice; the interchangeability of each other and one another had been established centuries before Ussher or somebody even earlier thought up the rule.

Fowler 1926 notes that some writers follow the rule but goes on to state that “the differentiation is neither of present utility nor based on historical usage; the old distributive of two as opposed to several was not e[ach], but either; & either other, which formerly existed beside e[ach] o[ther] & one another would doubtless have survived if its special meaning had been required.” Even Fowler’s high reputation among usage commentators has not convinced those to whom the rule is dear; many still prescribe it. A few examples may illustrate the rule’s baselessness:

Sixteen ministers who meet weekly at each other’s houses -Samuel Johnson, Life of Swift (in Brown 1851)

Most of whom live remote from each other -Noah Webster, Essays (in Literary Digest, 5 July 1924)

The spouse aspires to an union with Christ, their mutual love for one another -chapter bloss, Canticle of Canticles (Douay Version), 1609

Two negatives in English destroy one another -Lowth 1763

It is a bad thing that men should hate each other; but it is far worse that they should contact the habit of cutting one another’s throats without hatred -T.B. Macaulay (in Webster 1909)

...and the charity of every one of you all toward each other abundeth -2 Thessalonians 1:3 (AV). 1611

...he and Bikki were extremely jealous of one another -Stella Gibbons, Cold Comfort Farm, 1932

...and there is a chain of witnesses who confirm each other -G. K. Chesterton, “The Oracle of the Dog.” 1923

...Janet and Marcia would, by way of greeting, neigh at one another -John Updike, Couples, 1968

They had been marrying one another for so many centuries that they had bred into themselves just the qualities, ignorance and idiocy, they could least afford. At the funeral of Edward VII in London they had pushed and shoved and elbowed each other like children for places in the cortege -E. L. Doctorow, Ragime, 1976

A few commentators believe the rule to be followed in “formal discourse.” This belief will not bear examination:Samuel Johnson’s discourse is perhaps the most consistently formal that exists in English literature, and he has been cited in violation of the rule.

We conclude that the rule restricting each other to two and one another to more than two was cut out of the whole cloth. There is no sin in its violation. It is, however, easy and painless to observe if you so wish.

2. Wilson Follet, Erik Wensberg의 "Modern American Usage"에도 이러한 오류를 간략하고 정확하게 꼬집고 있다.

Each other was once each the other, and many believe that it still implies only two, and that one another suggests three or more. But usage has by now made the phrases interchangeable-without affecting their exclusive allegiance to single items. (p.110)

3. Bryan A. Garner의 "Garner's Modern American Usage"에서도 이 낡은 규칙은 잘 지켜지지 않는다고 말한다.

Usage authorities have traditionally suggested that each other should reter to two people or entities, one another to more than two. Yet this 19th-century rule has often been undermined in the literature on usage. (p.287)

4. Michael Swan의 Practical English Usage를 보면 "each other and one another" 라는 타이틀 아래에 등장하는 "1번" 소제목이 "No difference"이다. 그리고 이 둘은 같은 의미라고 되어 있다.

171. Each other and one another

1. no difference

Each other and one another mean the same.

ex) Ann and I write to each other / one another every week.

Each other is more common than one another, especially in an informal style. (p.153)

5. Collins Cobuild English Usage를 보면, each other와 one another를 둘 다 "two or more"의 같은 의미로 소개하고 있다. 그리고 똑같은 의미로 each other와 one another를 사용하는 예를 보여준다.

You use each other or one another when you are talking about actions or feelings that involve two or more people together in an identical way. For example, if Simon likes Louise and Louise likes Simon, you say that Simon and Louise like each other or like one another. (p.153)

6. Martin Steinmann and Michael Reller의  "Good Grammar Made Easy"를 보면 each other와 one another는 동의어라고 되어 있다.

each other, one another : Both are SAWE(Standard American Written English) and synonymous. (p.115)

7. Eastwood의 Oxford Learner's Grammar를 보면 둘 사이에 each other와 one another를 쓰고 같은 의미라고 설명하고 있다.

Each other / one another

Look at this example.

Jane and Amy mailed each other. jane and Amy mailed one another.

These sentences both mean that Jane mailed Amy and Amy mailed Jane. The mail went in both directions.

(p.244)

이제부턴 사전도 찾아보자. 

8. Cambridge English Dictionary 에서 "each other"의 정의를 찾아보면 "둘 또는 그 이상"이라고 되어 있다. 그리고 one another의 정의를 찾아보면 "each other"라고 되어 있다.

each other : "used to show that each person in a group of two or more people does something to the others"

one another : "each other"

9. Macmillan English Dictionary에서 "one another"를 찾아보면 분명히 "two or more"라고 되어 있다. 그리고 예문중에 주어를 "two"로 하는 예문이 소개되어 있다.

one another : "used for saying that each of two or more people does the same thing to the other person or the other people, or has the same relationship with them"

ex) The two of them sat facing one another.

10. Longman English Dictionary에서 "each other"를 찾앚보면 "two or more"라고 되어 있다. 그리고 "one another"를 찾아보면 정의를 "each other"라고 해 놓고, 예문에 분명히 "둘"이 등장한다.

* each other : used to show that each of two or more people does something to the other or others

* one another

= each other

Liz and I have known one another for years.