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We give an elementary criterion for the norm of the fundamental unit$\varepsilon_K$ of $K=\mathbb{Q}(\sqrt M)$, $M$ square-free. More precisely, if$\varepsilon_K = a+b\sqrt M$, $a,b \in \mathbb{Z}$ or $\frac{1}{2}\mathbb{Z}$,its norm ${\bf S}_K$ only depends on $m := {\bf gcd} \big(\frac{a+1}{{\bfgcd}(a+1,b)}, M\big)$ and $m' := {\bf gcd} \big(\frac{a-1}{{\bf gcd}(a-1,b)},M\big)$ as follows when $-1$ is a global norm: ${\bf S}_K = -1$ if and only if$m=m'=1$ (resp. $m=m'=2$) for $M$ odd (resp. even) (Theorems 1.1 or 2.4).

New characterization of the norm of the fundamental unit of $\mathbb{Q}(\sqrt M)$