I’m used to the modern American system, where challenges of jurors for bias are decided by the judge. But Blackstone’s Commentaries reports that in 1765, the English rule was that such challenges were to be decided by two of the already impanelled jurors (or by specially appointed “indifferent persons named by the court” if two jurors had not yet been impanelled). And this practice persisted, at least in parts of the U.S., through the first decades of the 1800s (see, e.g., this New York case from 1819).
I had known that the Framing era in many ways left more to the decision of jurors, as opposed to judges, than our current system does. I just hadn’t known of this particular way.
UPDATE: Two commenters report that the system I describe is still used in Canada, and this 1999 article confirms that, and reports further.