FEW RULES COULD be clearer, in theory, than “Thou shalt not kill.” Yet even that commandment is subject to interpretation and exceptions (war, self-defense). And so the history of law-making is studded with well-meaning prohibitions — Prohibition itself, to name one — that were straightforward in principle but rather more complicated and occasionally unworkable in practice.

Is the Volcker Rule the latest example? When enacted in 2010 as part of the Dodd-Frank financial reform law, the rule was intended to ban commercial banks from investing their own money in hedge funds and other speculative businesses. The rule’s purpose, according to its eponymous advocate, former Federal Reserve chairman Paul Volcker, is to keep banks, whose deposits are federally insured, from taking excessive risks based on the funding advantage that they get from implied government support.

Mr. Volcker argued that this widespread practice was not only unfair but systemically destabilizing. Better, he said, to reestablish a clear line between commercial banks, which do basic financial intermediation — lending savers’ deposits to businesses — and the uninsured investment banks, which may take higher risks in pursuit of higher rewards. That line was established under the New Deal but repealed in recent times.

Sounds simple, except that everything about the Volcker Rule, from its rationale to the precise definition of such seemingly obvious terms as “hedge fund,” is open to debate. The financial crisis did not begin with the collapse of a commercial bank but with the downfall of Lehman Brothers, an investment bank. Sometimes the commercial banks’ trading is best described as necessary risk-mitigation or client service, rather than mere profit-maximization.

Dodd-Frank itself took account of these nuances by allowing exceptions to the Volcker Rule. The federal banking agencies recently published a proposed implementing rule — all 298 pages of it — that elaborates on those exceptions. As the draft candidly notes, putting the Volcker Rule into operation “often involves subtle distinctions that are difficult both to describe comprehensively within regulation and to evaluate in practice.” The agencies estimate that 10,000 U.S. banks may eventually spend a combined 1.8 million hours a year complying with the rule. Of course, that could change if the rule evolves between now and the final version, due next year. Evolution seems probable, given that regulators posed 383 issues for public comment by Jan. 13.

No wonder banks and financial-reform activists are unhappy: The former decry the proposed regulation’s costs, and the latter claim banks lobbied the Volcker Rule to death. Both have a point: The proposal’s complexity is inherent in the rule-making process and a result of the many compromises regulators struck to appease banks and other constituencies.

Still, the rule is hardly unjustified: Though banks’ trading losses did not start the financial crisis, they probably intensified it. Nor is the rule toothless. Most of the argument now is over finer points, not the core definition of proprietary trading, which some major banks had already begun to exit in anticipation of the rule. The apparent slump in financial-sector earnings may indeed reflect Dodd-Frank’s impact. U.S. commercial banks will soon find it harder to trade on their own accounts, and formerly high-flying proprietary trading desks may migrate abroad.

Let them migrate. Of course, banks will pass the costs of the Volcker Rule on to their customers — in higher fees or reduced services, or both. But financial stability is a public good, not a free lunch. A well-designed Volcker Rule could help prevent or mitigate crises like the one the world lived through in 2008 — and from which it has not fully recovered.