We have mothers-in-law, fathers-in-law, sons- and daughters-in-law, sisters- and brothers-in-law. But what should you call the parents of your child’s spouse?
English, alas, has no specific term. You might say “my daughter-in-law’s parents,” or more vaguely, “the in-laws.”
The Oxford English Dictionary traces the first usage of “brother-in-law” and such to around 1300; it meant a relative not by blood, but by the canon law. Under that churchly view, the person was the equivalent of a blood brother or sister, and thus off-limits for marriage. What we now call “step-” relatives (stepfather, stepchildren, stepsister, etc.) were also in the “in-law” category, in language as well as in law, as far as intermarriage was concerned. From a language point of view, the “step-s” have their own names now. From a marriage point of view . . . well, you all know Woody Allen, right?
“In-laws” as a general term is a back-formation, a shortened term arising from a longer one. The OED quotes an 1894 magazine saying Queen Victoria coined that “happy phrase,” which “is often not very apt to promote happiness.”
But the phrase “in-laws” almost always means the relationship of your spouse’s family to you, or the relationship of your family to your spouse, and not the relationship between the sets of parents.
In Yiddish, your parents and your spouse’s parents would be machatunim (approximate pronunciation: mah-cha-tuh-num, with the “cha” rolled in the back of your throat). In Spanish, they would be consuegros, roughly, “co-in-laws.”
Perhaps it’s time for English to adopt such a term. Of course, we’d have to add “step-machatunim” or “civil-union-consuegros” to account for all the variations in relationships we have these days. Any suggestions?