The Morse tree: a variable-length code a century before Huffman
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Morse code is usually presented as a table: a letter, and beside it the dots and dashes. In that form it looks like an arbitrary list, and all you can do with it is memorise it.
It is in fact a binary tree, and in that form you can see the engineering decision taken in the 1840s: the frequency of a letter sets its depth. E is one dot, T one dash, because those are the commonest letters in English. Q, J and Z sit all the way down on the fourth level. Alfred Vail arrived at this not by theory but by counting: he counted the letters in the type cases of a local newspaper.
A variable-length code, allocated by symbol frequency. A hundred-odd years before Shannon and Huffman explained why that works.