Let $U\subset\mathbb R^n$ be open, let $T:U\to U$ be a $C^1$ diffeomorphism from $U$ onto $U$, and let $JT_x$ denote the Jacobian matrix of $T$ at $x\in U$. If
\begin{align*}
|\det JT_x|=1
\end{align*}
for every $x\in U$, then $T$ preserves the restricted [Lebesgue measure](/page/Lebesgue%20Measure) $\mathcal L^n|_U$.