While we’re all busy trying to remember that the plural of mother-in-law is mothers-in-law, not mother-in-laws, we often forget to ask a far more interesting question: Why do we call them in-laws in the first place?
You might assume it’s because your spouse’s family members are related to you by law, not by blood—but the law in question has nothing to do with the marriage license your officiant ships off to the county clerk. According to the Oxford English Dictionary, in-law refers to canon law, a church’s set of rules and regulations that covers, among many other things, which relatives you’re prohibited from marrying. Since the earliest known mention of the term in English is brother-in-law from the 14th century, it was most likely citing the canon law of the Catholic Church (as the Church of England wasn’t founded until the 16th century).
At its inception, in-law was specifically used to describe any non-blood relative that the church forbade you from marrying if your spouse died: your spouse’s siblings, parents, and children, and even your own stepsiblings, stepparents, and stepchildren. So father-in-law, as The Word Detective explains, could’ve either meant your spouse’s father, or your mother’s new husband. But by the late 19th century—at which point the Church of England and other Protestant faiths had established their own canon laws with varying marriage rules—the colloquial definition had expanded to include all spousal relatives, and in-laws became a standalone phrase. The earliest written mention of it comes from an 1894 article in Blackwood’s Edinburgh Magazine, which states that “the position of the ‘in-laws’ (a happy phrase which is attributed with we know not what reason to her Majesty, than whom no one can be better acquainted with the article) is often not very apt to promote happiness.”
In other words, tension between people and their in-laws has been around for as long as the phrase itself. If that's part of what brought you here in the first place, here are 12 pieces of 19th-century advice for dealing with them.
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