  Element-by-element we have combined the optical components in the three 2MASS
cameras, and incorporated detector quantum efficiency curves and site-specific
atmospheric transmissions, to create three relative spectral response curves
(RSRs). We provide 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 spectra for the purpose of using 2MASS data to support the development of
absolute calibrators for IRAC 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 1.2-35 um with our predictions, thereby
deriving 2MASS "zero point offsets" from the ensemble. These offsets are the
final ingredients essential to merge 2MASS $JHK_{s}$ data with other absolutely
calibrated bands and stellar spectra, and to support the creation of faint
calibration stars for SIRTF.
