Unity does not necessarily mean that the whole letter must be devoted to one point. A paragraph and even a sentence must have this quality of unity as much as the entire letter. And the paragraphs, each unified in itself, may bring out one point after another that will still allow the letter to retain its hang-together.
In the letter quoted, not even the individual sentence retained unity. This writer might have presented all his points and maintained the unity of his letter, had he brought out and simplified one point in each paragraph:
First: The size of the catalogue as an indication of the large stock carried by the house and the convenience afforded in buying.
Second: The quality of the machines; the care exercised in their assembling; the guarantee of the test, and the assurance that this gives the far-away purchaser.
Third: Promptness in filling orders; what this means to the buyer and how the house is organized to give service.
Fourth: The desire to enroll new customers; not based solely on the selfish desires of the house, but on the idea that the more customers they can get, the bigger the business will grow, which will result in better facilities for the house and better service for each customer.
And now, giving a unified paragraph to each of the ideas, not eliminating subordinate thoughts entirely, but keeping them subordinate and making them illuminate the central thought—would build up a unified, logical letter.
In the arrangement of these successive ideas and paragraphs, the third element in the form is illustrated—continuity of thought. Put a jog or a jar in the path of your letter and you take the chance of breaking the reader's attention. That is fatal. So write a letter that the reader will easily and, therefore, unconsciously and almost perforce, follow from the first word to the last—then your message reaches him.
How to secure this continuity depends on the subject and on the prospect. Appealing to the average man, association of thoughts furnishes the surest medium for continuity. If you lead a man from one point to another point that he has been accustomed to associating with the first point, then he will follow you without a break in his thought. From this follows the well-known principle that when you are presenting a new proposition, start your prospect's thoughts on a point that he knows, which is related to your proposition, for the transition is easiest from a known to a related unknown.
An insurance company's letter furnishes a good example of continuity of ideas and the gradual increasing strength in each paragraph: