  We present high spectral resolution optical observations of comet
21P/Giacobini-Zinner from six nights in 2018. The observations were obtained
with the Tull coude spectrograph on the 2.7m Harlan J. Smith Telescope of
McDonald Observatory. This comet's spectrum shows strong depletions in C$_2$,
C$_3$, CH, and NH$_2$ relative to CN. We explore what it means for a comet to
be depleted and show that all of the species are present in the spectrum at
similar relative line intensities within a a molecular band compared with a
typical comet. The depletions represent a much lower abundance of the species
studied.
