Earlier this month I noted that despite sporadic attacks on the present Supreme Court as supposedly gripped by a result-oriented and pro-business majority, “much of its work [in business law] consists simply of trying to keep the law on a logically coherent and predictable course,” often by unanimous vote. Today we can add another example: a unanimous Court (with Justice Sotomayor withholding consent from one footnote) ruled that U.S. Steel does not owe workers back pay for time spent donning and doffing protective gear in a context where the union representing the workers had specifically bargained away any right for them to be paid for that time.

If it seems bizarre for employees to claim a right to pay that their union has elected to waive during contract negotiations, read on. Like some others before it, this case illustrates a tension I described in my book The Excuse Factory between the old and mostly stagnant field of labor law — in which unions and their strike threat had been envisaged as the driving and potent force, and progress is measured by contracts for future higher pay — and the newer, perennially self-energizing employment law, in which private attorneys and their lawsuits act as the driving force, with the goal being big backward-looking settlements and the associated attorneys’ fees. So the first point about Sandifer v. U.S. Steel Corp. is that the steelworkers’ union was not the plaintiff, and that we shouldn’t assume unions necessarily wish suits of this kind to succeed.

Private employment-law attorneys do well enough from discrimination and harassment law, but their fastest-growing field of activity in recent years has been wage-and-hour law. Together with several associated statutes, the New Deal-era Fair Labor Standards Act (FLSA) creates many openings to sue in class or collective actions over large retroactive pots of pay for allegedly mischaracterized work — salary vs. hourly-wage, tipped vs. off-tip, employee vs. independent-contractor, and many others. That a particular company policy was well explained to workers at the time, and met with no objection, is no defense, since contracting around the rules is mostly not allowed. For example, an up-and-coming theme in wage-hour lawsuits is that employees should be able to claim retroactive on-the-clock pay for time spent away from the workplace using (or simply being available for) company cellphones, pagers, or email — a form of liability to which many employers have begun reacting by forbidding use of company cellphones or email outside work hours.

While many of the dictates of wage-hour law are appallingly obscure — a quarter century ago Judge Frank Easterbrook eloquently decried the high cost of its tendency to leave the fact of liability uncertain until long after employers have acted — Congress had actually come very near addressing the question at issue in 1949 when it enacted a relatively narrow legislative fix declaring that it would be up to unions to decide whether to seek or waive pay for time spent “changing clothes.”

This still left a crack of ambiguity wide enough to try to slip a suit through (the legal, if not the apparel, kind). Lawyers for Sandifer argued that the task of donning metal-tipped boots, flame-retardant jackets and leggings, and other steel-mill gear did not qualify as “changing clothing” because, among other reasons, many of the protective garments were donned on top of (rather than substituting for) street clothes. That meant, they argued, that the union had no power to bargain away the entitlement to the time, and Sandifer and others could seek back pay. The Court unanimously disagreed. It conceded that some types of technical gear, such as safety goggles and wearable electronics, will not qualify as “clothing,” but the overall activity of donning steel-mill protection still more closely resembles “changing clothes” than anything else.

So there’s a bit of clarity for the law, at long last. Now if only Congress felt any responsibility to clarify — or better yet, move to repeal — the hundred other ambiguous demands of wage-hour law.