Element by element, we have combined the optical components in the three cameras of the Two Micron All Sky Survey (2MASS), and incorporated detector quantum efficiency curves and site-specific atmospheric transmissions, to create three relative spectral response curves (RSRs). We provide the absolute 2MASS attributes associated with ``zero magnitude'' in the JHKs bands so that these RSRs may be used for synthetic photometry. The RSRs tie 2MASS to the ``Cohen-Walker-Witteborn'' framework of absolute photometry and stellar spectra for the purpose of using 2MASS data to support the development of absolute calibrators for the Infrared Array Camera and pairwise cross-calibrators between all three SIRTF instruments. We examine the robustness of these RSRs to changes in water vapor within a night. We compare the observed 2MASS magnitudes of 33 stars (converted from the precision optical calibrators of Landolt and Carter-Meadows into absolute infrared calibrators from 1.2 to 35 μm) with our predictions, thereby deriving 2MASS ``zero-point offsets'' from the ensemble. These offsets are the final ingredients essential to merge 2MASS JHKs data with our other absolutely calibrated bands and stellar spectra, and to support the creation of faint calibration stars for SIRTF.