A matrimonially-minded listener asked about the wording in wedding ceremonies. His curiosity was piqued by the phrase 'til death do us part. A more likely phrase, according to our correspondent, is "'til death does us part."
We agree: "'til death does us part" does sound more natural, especially if you rearrange the words into a more traditional order: "'til death does part us," which is what the phrase actually means, right?

Wrong. When the ceremonial phrase 'til death do us part was coined these many centuries ago, the English verb had a different meaning. Back then, do carried the (now-archaic) function of an auxiliary verb meaning "to cause or make."

Need an example in context? Back in the 16th century, English speakers might say to do him die. Its meaning? "To make him die, or to put him to death." This meaning of do is exactly the same one preserved in the phrase 'til death do us part, which literally means, "'til death make us part."

Now let's get back to our original question: why do instead of does? Because the verb is being used in the good old-fashioned subjunctive, the mood reserved for hypothetical situations.