In dealing with rules of a procedural type lawyers often make a distinction which is referred to as that between ‘mandatory’, and ‘directory’ rules. (The terms ‘imperative’, ‘absolute’, ‘obligatory’, ‘compulsory’, and ‘peremptory’, have sometimes been used instead of ‘mandatory’.) The distinction has to do with the effect of breach of a rule on the process to which it relates. Very broadly, mandatory rules are those procedural rules the breach of which necessarily invalidates the process to which they relate, while directory rules are procedural rules the breach of which does not necessarily have this effect. This distinction has existed in the common law for about three hundred years, yet, so far as I can discover, no serious attempt has ever been made to explore its logical basis. That is surprising, for on its face the distinction is a puzzling one.