COS has two channels: a Far-Ultraviolet (FUV) channel that covers the 1150 - 1775 Å wavelength region, and a Near-Ultraviolet (NUV) channel that covers the 1700 - 3200 Å wavelength region. Each channel has its own detector and selection of gratings. The two channels cannot make parallel observations, as the primary optics select mechanism either rotates a grating into position for FUV spectroscopy or else an optic that feeds the NUV channel. An observer will specify a target, its coordinates, an exposure time, and then select which channel (COS/FUV or COS/NUV), which aperture, and which grating to use. Finally, the observer will specify the central wavelength of the exposure. The central wavelength will be chosen from a table of ``legal'' pre-set values designated for each grating. These values will allow any region of interest in the entire 1150 - 3200 Å wavelength region to be covered. The NUV gratings, in particular, are flat gratings mounted in a collimated beam that are meant to be scanned in order to achieve wide wavelength coverage (due to the relatively small format of the NUV detector). The FUV gratings, on the other hand, each cover approximately 300 Å per exposure. They will normally operate at a ``standard'' central wavelength setting. However, some wavelengths that fall within the FUV ``detector gap'' are lost with these standard settings, so the FUV gratings will also have a limited set of alternate central wavelength positions that make it possible to shift the spectrum on the detector in order to recover the needed wavelengths.