Interesting; this article says the rewrite rule is <i>F -&#62; F+F-F-F+F</i> , but Wikipedia says <i>F → F−F++F−F</i> . I think Wikipedia's is right, as (for a beginning) there are only four line segments in step N+1 for each line segment in step N. Take out the F between the minus signs in this article's rule, and it would be correct ( <i>F -&#62; F+F--F+F</i> ), allowing for mirroring of the Wikipedia formula.