answer:I did when I was a youngster, age 10 or younger. The structure was essentially basic English grammar with invented vocabulary substituted for ordinary English words. It wasn’t enough that I made it up; I made my friends learn it as I went along, drilled them in it, and insisted that we speak in it at least part of the time, especially when parents were present. It bugged the hell out of the parents. It never actually got very complex, and after a while the fun wore off and we forgot about it. It never crossed my mind that language wouldn’t have the same fascination for others that it had for me. And I was always the one with the ideas for games, so they just went along with whatever I came up with. In high school I developed my own writing system. It was strictly English, but with my own characters and with certain special conventions so that it was mostly but not entirely a straight symbol substitution. That was to defeat a purely cryptogram-solving approach to breaking it. I became practiced enough in it to be able to read and write in this code as fluently as in the Roman alphabet. Also I created symbols with a kind of stylistic consistency so that it had an integrity of design and was very pretty to look at. I can no longer read the diaries I wrote in it, but I’d be able to decode it pretty quickly if I sat down and worked on it. I know this is different from what you’re doing, @DominicX. You are probably attempting something more like what Tolkien was up to, correct? But it isn’t altogether unrelated.