  The C II 133.5 nm multiplet has been observed by NASA's Interface Region
Imaging Spectrograph (IRIS) in unprecedented spatial resolution. The aims of
this work are to characterize these new observations of the C II lines, place
them in context with previous work, and to identify any additional value the C
II lines bring when compared with other spectral lines. We make use of wide,
long exposure IRIS rasters covering the quiet Sun and an active region. Line
properties such as velocity shift and width are extracted from individual
spectra and analyzed. The lines have a variety of shapes (mostly single-peak or
double-peak), are strongest in active regions and weaker in the quiet Sun. The
ratio between the 133.4 nm and 133.5 nm components is always less than 1.8,
indicating that their radiation is optically thick in all locations. Maps of
the C II line widths are a powerful new diagnostic of chromospheric structures,
and their line shifts are a robust velocity diagnostic. Compared with earlier
quiet Sun observations, we find similar absolute intensities and mean line
widths, but smaller red shifts; this difference can perhaps be attributed to
differences in spectral resolution and spatial coverage. The C II intensity
maps are somewhat similar to those of transition region lines, but also share
some features with chromospheric maps such as those from the Mg II k line,
indicating that they are formed between the upper chromosphere and transition
region. C II intensity, width, and velocity maps can therefore be used to
gather additional information about the upper chromosphere.
