  We characterize Radon measures $\mu$ in $\mathbb{R}^{n}$ that are
$d$-rectifiable in the sense that their supports are covered up to
$\mu$-measure zero by countably many $d$-dimensional Lipschitz graphs and $\mu
\ll \mathcal{H}^{d}$. The characterization is in terms of a Jones function
involving the so-called $\alpha$-numbers. This answers a question left open in
a former work by Azzam, David, and Toro.
