Paul Halmos states:
It is frequent in mathematics that every instance of a concept of seemingly great generality is in essence the same as a small and concrete special case.
What are examples of such: Small, concrete, individual special cases that turn out to be unexpected archetypes covering wide ranges of unexpected generality?
One example is the Cantor Set. This seems like a very unique, artificial, one-off construction. Until you learn the topology showing that huge ranges of sets are equivalent to or built from it (those who know more topology than me can fill in the details.)
What other examples exist of "small and concrete special cases" that turn out to capture "every instance of a concept of... great generality"?