Intended audience: data compression geeks. The Burrows-Wheeler transform takes a string as an input and rearranges its characters, grouping them by context. It is invertible with 𝒪 ( 1 ) additional input, and together these properties give it a place in data compression and genome alignment. What Wikipedia doesn’t tell you is that there are actually two variants of BWT, with subtly different performance characteristics and simplicity. These differences seem to be largely undocumented, so this post is me trying to make sense of it.