The Bertrand-Chebyshev Theorem was first postulated by Bertrand in $1845$. He verified it for $n < 3 \, 000 \, 000$.
It became known as Bertrand's Postulate.
The first proof was given by Chebyshev in $1850$ as a by-product of his work attempting to prove the Prime Number Theorem.
After this point, it no longer being a postulate, Bertrand's Postulate was referred to as the Bertrand-Chebyshev Theorem.
In $1919$, Srinivasa Ramanujan gave a simpler proof based on the Gamma function.
In $1932$, Paul Erdős gave an even simpler proof based on basic properties of binomial coefficients. That proof is the one which is presented here.