We derive the triply differential spectrum for the inclusive rare decay $B\to X_s{\ell }^{+}{\ell }^{-}$ in the shape-function region, in which $X_s$ is jetlike with $m_X^2\lesssim m_b{\Lambda }_{\mathrm{QCD}}$. Experimental cuts make this a relevant region. The perturbative and nonperturbative parts of the matrix elements can be defined with the soft-collinear effective theory, which is used to incorporate ${\alpha }_s$ corrections consistently. We prove that, with a suitable power counting for the dilepton invariant mass, the same universal jet and shape functions appear as in $B\to X_s\gamma$ and $B\to X_u\ell \overline{\nu }$ decays. Parts of the usual ${\alpha }_s(m_b)$ corrections go into the jet function at a lower scale, and parts go into the nonperturbative shape function. For $B\to X_s{\ell }^{+}{\ell }^{-}$, the perturbative series in ${\alpha }_s$ are of a different character above and below $\mu =m_b$. We introduce a “split matching” method that allows the series in these regions to be treated independently.

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