4100. Let the direction be written very plain; this will save the postman trouble, and facilitate business by preventing mistakes.
4101. At the head of your letter, in the right-hand corner, put your address in full, with the day of the month underneath; do not omit this, though you may be writing to your most intimate friend three or four times a day.
4102. What you have to say in your letter, say as plainly as possible, as if you were speaking; this is the best rule; do not revert three or four times to one circumstance, but finish up as you go on.
4103. Let your signature be written as plainly as possible (many mistakes will be avoided, especially in writing to strangers) and without any flourishes, as they tend not to add in any way to the harmony of your letter.
4104. A letter of introduction is to be enclosed in an unsealed envelope.