Our alphabet code, entered in a notebook when I was 12.
When I was a kid, back in the dark days before Encarta and Wikipedia and all that, we had a couple of sets of encyclopedias. They were great sources of knowledge, as well as interesting snapshots of a given period in time--one set dated from the 60s, another was slightly more recent, though still out of date, so the science items in particular were occasionally unintentionally comical. (You can imagine the example photos they had in the article about "Computer," for example...)
My brother John and I spent many a rainy day or winter evening with those books, either tracing themes from one volume to another, or just reading straight through. Sometimes I'd take a volume to bed with me and fall asleep reading about aardvarks and Albania and the Appalachian Trail. I also liked studying the very first page of those volumes, where they discussed the history of whatever-letter-it-was.
Now...like most kids, we were also captivated by secret codes. Who wasn't? We came up with various forms of encryption, like the old standard of using a key word to come up with a 26 number alphabet code. Remember that? Basically, you'd pick a random word with no repeated letters, write that out, followed by the remaining letters of the alphabet, and write the numbers 1 through 26 by them. Unless the recipient knew the keyword, it was hard to figure out which numbers meant what.
But like I said, I also found those historical predecessors to the current alphabet very interesting, and somewhere along the line, we hit upon the idea of using those to make a simple to use and intuitive "code" to use amongst ourselves. It was pretty easy to break, but easy to memorize and fun. Some letters had common predecessors or no very interesting variations, so we had to make up a few letter shapes, but most came straight from older forms. I kept journal entries in code some of the time back then, and even as an older teenager away from home used to write letters to my brother using these letters. Kinda fun.
What sort of codes have you played around with? What did you use them for, and with whom?