How it works: After a page of writing had been completed, often when both sides had been filled, the write turned the page 90 degrees and continued writing, adding a second layer of text. To decipher the letter, one simply must read in the direction of the text. The reader's eye tends to naturally tune out the perpendicular lines and can follow what was written.
Although cross-writing was used, it wasn't always advised, especially later in the century.
"When you get to the end of a notesheet, and find you have more to say, take another piece of paper—a whole sheet, or a scrap, as the case may demand: but whatever you do, don’t cross! Remember the old proverb 'Cross-writing makes cross reading.' 'The old proverb?' you say, inquiringly. 'How old?' Well, not so very ancient, I must confess. In fact, I’m afraid I invented it while writing this paragraph! Still, you know, 'old' is a comparative term. I think you would be quite justified in addressing a chicken, just of of the shell, as 'Old boy!' when compared with another chicken, that was only half-out!"
~ Lewis Carroll on How to Write a Letter (Rule #9), 1894
Examples of Cross-writing:
Mrs. F. L. Bridgeman to Fanny West, December 15, 1837 - In this image and the next the fold of the letter to create the "envelope".
"My Dear Herb
I have treated you very badly in not writing but the truth is I have been so hard put for time that I have not been able to do so. Your letter reached me, about a month after it was written. I hope this will find you in Rome. Will you tell Samuel that I have not got time to write and that I am sorry to say I shall not be able to get abroad this year after all and so unless he can get to England, I shall not have the pleasure of seeing him. I suppose we shall see you up here next term at least at Oxford. You already know my eyes have been bad and have thrown me back considerably, I am very much..."
The crossed lines continue from the other side of the letter....
"feet deep here and I very nearly killed myself the other day up et..."
Read complete letter: http://www.flickr.com/photos/boston_public_library/2653156796/in/photostream/
Complete letter and images: http://www.archive.org/details/lettertoannewarr00west
"My dear Fox
I am dying by inches, from not having any body to talk to about insects: — my only reason for writing, is to remove a heavy weight from my mind, so now you must understand, what you will perceive before you come to the end of this; that I am writing merely for my own pleasure & not your's. — I have been very idle since I left Cambridge in every possible way and amongst the rest in Entomology. I have however captured a few insects, about which I am much interested: My sister has made rough drawings of three of them…"
And now that we are done intruding on the correspondence of others, you may like to puruse the following links on letter writing and postal service in the 18th and 19th centuries:
The Postal Service in 18th Century Britain: Letters and the Penny Post
The Victorian Revolution in Letter Writing
About Letter Writing in the 19th Century
When was the last time you wrote a full-length letter on paper and mailed it with a stamp? How many pages was it and did it require extra postage?