Michael Cohen’s testimony resumed Monday in Donald Trump’s criminal trial. But before the key witness retook the stand, a discussion between Judge Juan Merchan and the lawyers for each side signaled an issue whose importance rivals the ex-fixer’s testimony: jury instructions.

Surprising as that may sound, the mundane-sounding issue of jury instructions is critical in any case. This general principle holds true in the first criminal trial against a former U.S. president.

Monday morning’s discussion involved a potential defense witness who’s an election law expert, former Federal Election Commission chair Bradley A. Smith. Election law is relevant to this case generally because Trump is charged with falsifying business records in the first degree, a felony that requires prosecutors to prove that the falsification was done with intent to conceal or commit another crime. That other crime, prosecutors have indicated, is election law-related. (Trump has pleaded not guilty to all charges.)

But the potential problem with a witness testifying about law (as opposed to facts) is that Merchan is the one who instructs the jury on the law. So prosecutors don’t want any conflict between what a defense witness says and what the state thinks the law requires.

Conversely, the defense wants to present a view of election law that is favorable enough to Trump that it will create doubt about whether he intended to commit or conceal any election-related crimes. Prosecutors need to prove the charges beyond a reasonable doubt. Merchan has indicated that the defense expert, if called to the stand, can’t go into all the areas of election law that the defense wants him to. He told the lawyers to submit proposed jury instructions.

How Merchan ultimately instructs the jury on how to understand these somewhat complicated charges will be vital. Jury instructions are the lens through which jurors apply the evidence, including Cohen’s key testimony, to the question of whether Trump is guilty or not guilty. As we approach the end of the case and summations, no matter who else testifies, the battle over jury instructions could play a big role in which side wins the war of this trial.

Subscribe to the Deadline: Legal Newsletter for weekly updates on the top legal stories, including news from the Supreme Court, the Donald Trump cases and more.

This article was originally published on MSNBC.com