Consider the 24-element set $X = \{1,\ldots,24\}$, and the free vector space on it, identified with the power set of $X$. The the binary Golay code (sometimes called the extended binary Golay code to distinguish it from the perfect binary Golay code, which uses only 23 elements of $X$) has basis constructed as follows …

The automorphism group of the binary Golay code is the Mathieu group$M_{24}$, and the other Mathieu group are obtained as stabilisers of various sets in the Golay code. There is a unique central extension of the binary Golay code by $\mathbb{Z}/2$ which is not a group but a code loop, and can be used to construct the Monster group.