When a number is written in the margin of your theme, you are to turn to the article which corresponds to the number. Read the rule (printed in bold-face type), and study the examples. When an r follows the number on your theme, you are, in addition, to copy the rule. When an x follows the number, you are, besides acquainting yourself with the rule, to write the exercise of five sentences, to correct your own faulty sentence, and to hand in the six on theme paper. If the number ends in 9 (9, 19, 29, etc.), you will find, not a rule, but a long exercise which you are to write and hand in on theme paper. In the absence of special instructions from your teacher, you are invariably to proceed as this paragraph requires.

Try to grasp the principle which underlies the rule. In many places in this book the reason for the existence of the rule is clearly stated. Thus under 20, the reason for the rule on parallel structure is explained in a prologue. In other instances, as in the rule on divided reference (20), the reason becomes clear the moment you read the examples. In certain other instances the rule may appear arbitrary and without a basis in reason. But there is a basis in reason, as you will observe in the following illustration.

Suppose you write, "He is twenty one years old." The instructor asks you to put a hyphen in twenty-one, and refers you to 78. You cannot see why a hyphen is necessary, since the meaning is clear without it. But tomorrow you may write. "I will send you twenty five dollar bills." The reader cannot tell whether you mean twenty five-dollar bills or twenty-five dollar bills. In the first sentence the use of the hyphen in twenty-one did not make much difference. In the second sentence the hyphen makes seventy-five dollars' worth of difference. Thus the instructor, in asking you to write, "He is twenty-one years old," is helping you to form a habit that will save you from serious error in other sentences. Whenever you cannot understand the reason for a rule, ask yourself whether the usage of many clear-thinking men for long years past may not be protecting you from difficulties which you do not foresee. Instructors and writers of text books (impressive as is the evidence to the contrary) are human, and do not invent rules to puzzle you. They do not, in fact, invent rules at all, but only make convenient applications of principles which generations of writers have found to be wisest and best.