Legal directives fall along a spectrum between rules and standards, a distinction that has occupied jurisprudence far longer than the modern vocabulary suggests. A rule specifies its content in advance: a driver exceeding sixty-five miles per hour violates the statute regardless of road conditions or the reasonableness of her speed. A standard, by contrast, fixes only a background principle - drive reasonably - and defers to the adjudicator the task of giving that principle content in the particular case. The choice between the two forms, many theorists have argued, is not merely a matter of drafting technique but reflects a deeper allocation of authority: rules transfer discretion from the judge who applies the law to the legislator who enacts it, whereas standards do the reverse. The conventional case for rules rests on their predictability. Because a rule's content is settled before any dispute arises, citizens can order their affairs around it, and like cases are decided alike without dependence on the sympathies of a particular tribunal. Rules also economize on decision costs at the moment of application, since the adjudicator need only determine whether the triggering facts obtain. Yet these virtues are purchased at a price. A rule, being framed in advance and in general terms, will inevitably reach cases its drafters did not foresee and would not, on reflection, have wished to govern - the ambulance that exceeds the speed limit to save a life. The rule's very rigidity, its indifference to the equities of the individual case, is the source of both its strength and its characteristic injustice. Standards reverse this calculus. By postponing the specification of content until the facts are known, a standard can be tailored to circumstances the legislature could not have anticipated, and can thus avoid the over- and under-inclusiveness that afflicts rules. But the tailoring is done case by case, at considerable cost, and by officials whose judgments may diverge; predictability suffers, and the litigant is left to guess how a future court will assess the reasonableness of conduct already completed. It is tempting to conclude that the choice reduces to a trade-off between predictability and equity, to be struck differently as one or the other value predominates. That conclusion, I suggest, is too quick. The predictability of a rule is not a property of its form alone but of the surrounding practice: a nominal rule riddled with judicially crafted exceptions may be less predictable than a standard applied by a stable, disciplined bench. Conversely, a standard can acquire, through the accretion of decisions interpreting it, a body of concrete guidance that functions much as a rule would. The forms, in other words, tend over time to converge toward each other under the pressure of the values each initially slights. What appears at enactment to be a categorical election between rule and standard is better understood as the opening move in a longer process, one whose eventual character depends less on the drafter's initial choice than on the institutions charged with elaborating it.