WASHINGTON — The Supreme Court on Monday agreed to resolve a question that has vexed the lower federal courts since Congress enacted a law to narrow the gap between sentences meted out for offenses involving two kinds of cocaine.

Selling cocaine in crack form used to subject offenders to the same sentence one would get for selling 100 times as much in powder. The new law, the Fair Sentencing Act of 2010, reduced the disparity to 18 to 1, at least for people who committed their offenses after the law became effective on Aug. 3, 2010.

But what about people who committed their offenses before the statute came into force but were not sentenced until afterward?

For such defendants, Judge Terence T. Evans wrote in one of the pair of cases the Supreme Court agreed to hear, the law “might benefit from a slight name change: The Not Quite as Fair as it could be Sentencing Act of 2010 (NQFSA) would be a bit more descriptive.”