Examples of mathematical results discovered "late" What are examples of mathematical results that were discovered surprisingly late in history? Maybe the result is a straightforward corollary of an established theorem, or maybe it's just so simple that it's surprising no one thought of it sooner.
The example that makes me ask is the 2011 paper John Baez mentioned called "Two semicircles fill half a circle", which proves a fairly simple geometrical fact similar to those that have been pondered for thousands of years.


Theorem $\big($Sylvester-Gallai$\big):$ For any $n$ points in $\mathbb{R}^2$, not all collinear, there exists a line passing through exactly two of them.

$\rm{Proof}:$ Pick any $2$ points and draw a line $\ell$ through them: suppose a $3^{\rm{rd}}$ point lies on the line $($else we are done$)$ and pick the closest point $p\not\in\ell$ to the line, at a distance $\delta$ say. 
Of our $3$ points $\in\ell$ a pair lies on one side of $p:$ draw a line $\ell'$ through $p$ and the furthest of the pair from $p$. The distance $\delta'$ between $\ell'$ and the second point is $<\delta$.
$\qquad\qquad\qquad\quad$

The statement was proposed first by Sylvester in $1893$ and independently by Erdős in $1943\;($rather surprisingly, Erdős could not find a proof$)$. It was proven the following year by Gallai.
$\big[\mathbf{Note:}$ the statement can fail in other fields: for instance, in $\mathbb{P}^2(\mathbb{C})$ see the Hesse configuration$\big]$
