隔一場婚禮：最常見的兩條線
家族樹上只需要畫一條婚姻線，這個男生就到位了：不是你結婚把他帶進來，就是你姊妹結婚把他帶進來。日常說 brother-in-law，多半是這兩種。
noun · B2
The brother of the person you are married to. Your wife's brother, or your husband's brother, is your brother-in-law.
大伯／小叔／舅子
核心畫面
One way to hold this word: draw your family tree, then draw a wedding ring around two of the names. Every line inside the ring was made by a wedding, not by birth. A -in-law is a man who ends up standing where a would stand on that tree — same generation as you, close enough to sit at the family table — but the line that put him there is a marriage. The word never says whose marriage it was. That is why the same three cover your wife's , your sister's husband, and the man who married your wife's sister, and why speakers so often add one short sentence afterwards to say which line they mean.
可以想成一張家族樹：有些線是血緣畫的，有些線是婚禮畫的。brother-in-law 站的位置就是「兄弟」那一格，年…更多收起
可以想成一張家族樹：有些線是血緣畫的，有些線是婚禮畫的。brother-in-law 站的位置就是「兄弟」那一格，年紀同輩、坐同一桌吃飯，但把他放進去的是一場婚禮，不是同一對父母。這個字只說「同輩、男性、因婚姻而來」，從來不說是誰的婚禮，所以配偶的兄弟、姊妹的先生、連襟，全部共用這一個字。中文相反，大伯、小叔、舅子、姊夫、妹夫、連襟，一開口就把線路標死了。學這個字的重點，不只是背意思，而是習慣英文的作法：先用 brother-in-law 把人歸位，再補一句說明是哪一條線。From the word’s history
Middle English brother-in-lawe: brother plus -in-law, literally a brother by law rather than by birth. The -in-law part is exactly what the modern core image keeps: the line on the family tree was drawn by a marriage.
中古英文寫作 brother-in-lawe，就是 brother 加上 -in-law，字面上是「法律上的兄弟」。今天的核心畫面留下的正是 -in-law 這半邊：家族樹上那條線是婚禮畫的，不是血緣畫的。同一組還有 mother-in-law、son-in-law。
Wiktionary, entry 'brother-in-law' (kaikki:3f6f69a190036ae4:115795817:159412): 'From Middle English brother-in-lawe; equivalent to brother + -in-law.'
Two branches
隔兩場婚禮：連襟
同一張家族樹，只是要走兩條婚姻線才接得上：你娶進一個家，他也娶進同一個家。位置還是同輩男性，所以英文沒有換字，仍然叫 brother-in-law。
noun · B2
The man who is married to your husband's sister or your wife's sister. When two men marry two sisters, each one is the other's brother-in-law.
連襟