A few years ago I got tired of the standard Crown Royal bag of dice I carried to every gaming session. I thought it would be neat to make something unique. As I was doing a lot of modular knitting at the time, the idea of a knitted d20 came to mind right away. I went looking for patterns and discovered something interesting.
For the uninitiated, a d20 is a 20-sided die, made in the shape of a regular icosahedron. This Platonic solid is constructed from equilateral triangles. It's very easy to crochet shapes that have radial symmetry, like squares and regular hexagons, octagons, etc. This means that equilateral triangles are trivial to crochet, and there were several good plans for making a d20 that way. I really wanted to knit it, though, partly because I'm stubborn that way, but also because crochet in the round leaves lots of gaps and holes. That meant I'd probably have to line the bag, which I didn't feel like doing.
So I searched around some more for instructions on how to knit an equilateral triangle. I don't know if this is the page I found at the time, but it's certainly the method I used. Namely, casting on enough stitches for two sides, plus one stitch, and then knitting in garter stitch and making a double decrease three out of every four rows. Knitters know that you never decrease out of a decrease, so this involves some creative shifting of stitches, but it all works out.
Having knitted twenty little yellow equilateral triangles, I got some black yarn and a crochet hook and used slip-stitch crochet to seam them together. I made one group of five for the lid, another group of five for the bottom of the bag, and the remaining ten went together to make the "sides". I sewed in a purple zipper I had lying around, and then did simple backstitch embroidery to put the numbers on.
It's been quite the conversation piece at the D&D table!