We have observed with approximately the same spatial resolution HCN (J = 1-0), HNC (J = 1-0), their /sup 13/C isotopic species, and HC/sub 3/N (J = 2-1 and J = 10-9) toward six locations in the Taurus dark cloud complex, one dark cloud (L183) outside Taurus, and two warm clouds with associated H II regions. The column densities of HCN, HNC, and HC/sub 3/N are typically less than those of NH/sub 3/ by factors of 10/sup -1/-10/sup -3/. NH/sub 3/ is believed to be a tracer of the total H/sub 2/ abundance and a chemical precursor of the cyanogenated molecules. HNC is systematically about a factor of 10 more abundant than HC/sub 3/N. Limits on HCN column densities are given; upper limits were determined by limits on H/sup 13/CN emission, and lower limits were derived from the weakest hyperfine component, F = 0-1, of HCN (J = 1-0). The HNC/HCN column-density ratios lie in the range approx.3 to approx.10 in cold dark clouds. They are uncertain because HCN column densities are uncertain, but they are definitely greater than unity. The fact that this ratio does not deviate by large factors from unity can be understood in a gas-phase chemicalmore » network if ''chemical shuffling'' between HNC and HCN is important. Inclusion of reactions that convert HCN to HNC and vice versa forces the ratio HNC/HCN to a narrow range about unity if the chemical shuffling rates are fast relative to other HCN and HNC destruction rates.« less