As Miss Fishguard, in a conscientious determination to save Kitty the cost of receiving a second sheet, had crossed her lines closely, the task of deciphering the whole was very nearly impossible.Cotillion by Georgette Heyer, p. 283
The Royal Mail was generally very reliable, but for various reasons, including the necessary cost of countering the dangerous enterprise of footpads, the cost of postage was relatively high. In 1801, the rate was 1 penny per sheet, but by 1805 it had risen to 3 pence, and the responsibility for payment rested not with the sender but with the receiver (The Writer’s Guide to everyday Life in Regency and Victorian England, Kristine Hughes, p. 111). Envelopes were not used until later in the century, correspondents instead using a folded sheet of paper to enclose the letter. Often, to save money, they would use the letter itself as the envelope, leaving only the front blank for the address (direction), and writing and crossing their lines (as seen above) on the back and folded sides.
The only other way to save the cost of postage was to know a member of Parliament, each of whom were given “franking privilege,” which meant that their signature on an envelope was payment to the post office in full (though the post office may have grumbled over this circumstance more than once, as it was not actually reimbursed for any franked post until the 20th century).
The word “franking” comes from the latin root “francus,” meaning “free,” as we can see in the word “frankly” which means “freely.” So, frankly speaking, franking was just another example of unfair privilege given to those who didn’t need it, at the expense of those who did–unless the privileged used it on behalf of those less fortunate also, which did happen occasionally.