  We give a simple set of generators and relations for the Cremona group of the
plane. Namely, we show that the Cremona group is the amalgamated product of the
de Jonqui\`eres group with the group of automorphisms of the plane, divided by
one relation which is $\sigma\tau=\tau\sigma$, where $\tau=(x:y:z)\mapsto
(y:x:z)$ and $\sigma=(x:y:z)\dasharrow (yz:xz:xy)$.
