When Supreme Court justices sit down Friday to ponder whether they should rule on the constitutionality of Washington, D.C.’s strict gun control ordinance, they should be forewarned that they are stepping into a quagmire.

No, not the political quagmire over gun control. Another suddenly intense debate is enveloping the case — this one over what all those commas in the Second Amendment meant in late 18th-century America.