  Let $R$ be a commutative ring with identity and a fixed invertible element
$q^{\frac{1}{2}}$, and suppose $q+q^{-1}$ is invertible in $R$. For each planar
surface $\Sigma_{0,n+1}$, we present its Kauffman bracket skein algebra over
$R$ by explicit generators and relations. The presentation is independent of
$R$, and can be considered as a quantization of the trace algebra of $n$
generic $2\times 2$ unimodular matrices.
