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We present an exhaustive quantitative comparison of all the known extinction curves in the Small and Large Magellanic Clouds (SMC and LMC) with our understanding of the general behavior of Milky Way extinction curves. The R V -dependent CCM relationship of Cardelli, Clayton, and Mathis and the sample of extinction curves used to derive this relationship are used to describe the general behavior of Milky Way extinction curves. The ultraviolet portion of the SMC and LMC extinction curves are derived from archival IUE data, except for one new SMC extinction curve, which was measured using Hubble Space Telescope Space Telescope Imaging Spectrograph observations. The optical extinction curves are derived from new (for the SMC) and literature UBVRI photometry (for the LMC). The near-infrared extinction curves are calculated mainly from 2MASS photometry supplemented with DENIS and new JHK photometry. For each extinction curve, we give R V ¼ AðV Þ=EðB À V Þ and N(H i) values that probe the same dust column as the extinction curve. We compare the properties of the SMC and LMC extinction curves with the CCM relationship three different ways: each curve by itself, the behavior of extinction at different wavelengths with R V , and the behavior of the extinction curve Fitzpatrick and Massa fit parameters with R V . As has been found previously, we find that a small number of LMC extinction curves are consistent with the CCM relationship, but the majority of the LMC and all the SMC curves do not follow the CCM relationship. For the first time, we find that the CCM relationship seems to form a bound on the properties of all the LMC and SMC extinction curves. This result strengthens the picture dust extinction curves exhibit of a continuum of properties between those found in the Milky Way and the SMC bar. Tentative evidence based on the behavior of the extinction curves with dust-to-gas ratio suggests that the continuum of dust extinction curves is possibly caused by the environmental stresses of nearby star formation activity.

New broad-band UBV RI photoelectric observations on the Johnson-Kron-Cousins photometric system have been made of 202 stars around the sky, and centered at the celestial equator. These stars constitute both an update of and additions to a previously published list of equatorial photometric standard stars. The list is capable of providing, for both celestial hemispheres, an internally consistent homogeneous broadband standard photometric system around the sky. When these new measurements are included with those published in Landolt (1992), the entire list of standard stars in this paper encompasses the magnitude range 8.90 < V < 16.30, and the color index range −0.35 < (B − V ) < +2.30. Subject headings: stars: standard -photometry: broad-band -photometry: standardization to the upper right of the main sequence are SA98-L5 [(U − B) = −0.100; (B − V ) = +1.900] and SA110-273 [(U − B) = +1.000; (B − V ) = +2.527]. The associated photometric errors are large, especially for SA98-L5, and hence were not plotted in Figures 3 -9. Similarly, the (U − B) errors for the stars SA110-157, SA110-315 and SA110-273, large due to the small flux in the U-band, were not plotted in the relevant figures.

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