A triangle is a versatile knitting shape that can be used in a variety of ways, from making one big triangular shawl to joining smaller triangles together and making fun geometric patterns that can be incorporated into a larger knitted piece. The process involves a few small calculations and steps but once you've got the hang of it, your nimble fingers will be casting off triangles at ease.
Knit an inch-long swatch (sample) with your chosen wool and needles to calculate how many stitches are needed for the first row. Count the number of stitches per inch and multiply it by the number of inches you want the base of your triangle to be.
Cast on the number of stitches you calculated in the previous step and begin knitting the first row, which will form the base of your triangle.
Knit the first stitch of the second row as normal and then knit two stitches together. In the same way, when you get to the final three stitches of the same row, knit the second and third stitches from the end together and knit the final stitch normally.
Repeat this process for each row so that the rows decrease in size until there is only one stitch left on the needle, which will be the top point of your triangle. Cast off this stitch, cut the wool with scissors and weave the loose end through the stitch to secure it.