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© 2016. The American Astronomical Society. All rights reserved..We show that the mass fraction of neutral atomic gas (H i and He) in isolated local disk galaxies of baryonic mass is well described by a straightforward stability model for flat exponential disks. In the outer disk parts, where gas at the characteristic dispersion of the warm neutral medium is stable in the sense of Toomre, the disk consists of neutral atomic gas; conversely, the inner part where this medium would be Toomre-unstable, is dominated by stars and molecules. Within this model, only depends on a global stability parameter , where j is the baryonic specific angular momentum of the disk and s the velocity dispersion of the atomic gas. The analytically derived first-order solution provides a good fit to all plausible rotation curves. This model, with no free parameters, agrees remarkably well (±0.2 dex) with measurements of in isolated local disk galaxies, even with galaxies that are extremely H i-rich or H i-poor for their mass. The finding that increasing monotonically with q for pure stability reasons offers a powerful intuitive explanation for the mean variation of with : in a cold dark matter universe, galaxies are expected to follow , which implies the average scaling and hence , in agreement with the observations.