From Early Modern English brethren (plural of brother), from Middle English brethere, brether + -en (plural ending), alteration, due to Old English brēþer (dative singular), of Old English brōþor, brōþru (“brothers, brethren”). Compare German Brüder (“brothers, brethren”). More at brother. The vowel change from o to e is called umlaut.
- (archaic) Plural form of brother
- (figuratively) the body of members, especially of a fraternal, religious or military order
Usage notes 
The plural "brethren" is generally used for members of an organization, especially a religious body, whereas the plural "brothers" is used in the familial sense as well as for larger groups.
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