  For hydrodynamic model atmospheres, absolute lineshifts are becoming an
observable diagnostic tool beyond the classical ones of line-strength, -width,
-shape, and -asymmetry. This is the wavelength displacement of different types
of spectral lines away from the positions naively expected from the Doppler
shift caused by stellar radial motion. Caused mainly by correlated velocity and
brightness patterns in granular convection, such absolute lineshifts could in
the past be studied only for the Sun (since the relative Sun-Earth motion, and
the ensuing Doppler shift is known). For other stars, this is now becoming
possible thanks to three separate developments: (a) Astrometric determination
of stellar radial motion; (b) High-resolution spectrometers with accurate
wavelength calibration, and (c) Accurate laboratory wavelengths for several
atomic species. Absolute lineshifts offer a tool to segregate various 2- and
3-dimensional models, and to identify non-LTE effects in line formation.
