<i>RIVEN SPOILERS</i><p>I actually didn't play Riven until about 5 years ago, when it came out on iOS (I played Myst on CD as a kid). Learning the game's number system was a fascinating exercise that I don't think I've ever had in any other game.<p>Basically, in your explorations you eventually happen upon a small empty one-room building, which (if you look at the arrangement of desks) you can deduce is some sort of schoolhouse. In that room you'll find a mechanical wooden-and-string toy on a table. If you interact with it, the symbol on the base changes, and the toy performs some sort of action (I think dropping a weight) some number of times. From that you can deduce that the symbols are numbers, and the toy is for teaching what number each of the symbols represents.<p>If you're playing correctly, you've got some scratch paper handy (I used a drawing app) and you can write down each symbol and what number it corresponds to. And you'd better do this, because you're going to see a lot of important numbers in the game, and this is the only way they are written.<p>Towards the end of the game, you find a journal from one of the characters, which is written in English. However, when referring to the Age (the game's term for different worlds) that they've gone to, they use these numerals. If you've done your homework, you'll know that they've named their new world "The 233rd Age".