  The dust extinction curve is a critical component of many observational
programs and an important diagnostic of the physics of the interstellar medium.
Here we present new measurements of the dust extinction curve and its variation
towards tens of thousands of stars, a hundred-fold larger sample than in
existing detailed studies. We use data from the APOGEE spectroscopic survey in
combination with ten-band photometry from Pan-STARRS1, 2MASS, and WISE. We find
that the extinction curve in the optical through infrared is well characterized
by a one-parameter family of curves described by R(V). The extinction curve is
more uniform than suggested in past works, with sigma(R(V)) = 0.18, and with
less than one percent of sight lines having R(V) > 4. Our data and analysis
have revealed two new aspects of Galactic extinction: first, we find
significant, wide-area variations in R(V) throughout the Galactic plane. These
variations are on scales much larger than individual molecular clouds,
indicating that R(V) variations must trace much more than just grain growth in
dense molecular environments. Indeed, we find no correlation between R(V) and
dust column density up to E(B-V) ~ 2. Second, we discover a strong relationship
between R(V) and the far-infrared dust emissivity.
