The VLA's Modcomp computers support one special observing mode for single-antenna VLBI and two special observing modes for phased-array VLBI. As described below, these modes modify the phase corrections normally applied by the VLA's Modcomp computers to produce the appropriate signals to record for VLBI. Standard practice is to treat all VLBI programs at the VLA as as though they were VLA continuum programs, irrespective of whether the VLBI program is a spectral line or continuum one. However, it is also possible, and occasionally necessary, to run phased-array VLBI programs in VLA spectral line mode.