There is a puzzle which I remember from many years ago which can be summarised as
It is easy to arrange 20 items in five rows of four items, but how could you arrange just 10 items in five rows of four items?
The answer is this (spoiler-protected for those of you who want to try to solve it first):
which is all very clever, but what is the more general case of this lateral thinking puzzle?
[edit]: To clarify what I mean by "general case", I'm not so much asking whether there's a general case, but how to describe it, what values are possible, and how to find them.
For example, for an $n$-sided polygon (n>2), each vertex is in two rows of $2$ items, so the "trivial" general case is that $n$ items can be arranged in $n$ rows of $2$ if you only count the edges. However, for n>3, each vertex can be joined in $n-1$ rows of $2$ items, producing $\frac{1}{2}n(n-1)$ such rows in total.