  The mysterious "21 micrometer" emission feature seen almost exclusively in
the short-lived protoplanetary nebula (PPN) phase of stellar evolution remains
unidentified since its discovery two decades ago. This feature is always
accompanied by the equally mysterious, unidentified "30 micrometer" feature and
the so-called "unidentified infrared" (UIR) features at 3.3, 6.2, 7.7, 8.6, and
11.3 micrometer which are generally attributed to polycyclic aromatic
hydrocarbon (PAH) molecules. The 30 micrometer feature is commonly observed in
all stages of stellar evolution from the asymptotic giant branch (AGB) through
PPN to the planetary nebula phase.
  We explore the interrelations among the mysterious 21 micrometer, 30
micrometer, and UIR features in the Galactic and Magellanic Cloud of the 21
micrometer sources. We derive the fluxes emitted in the observed UIR, 21
micrometer, and 30 micrometer features from published ISO or Spitzer/IRS
spectra.
  We find that none of these spectral features correlate with each other. This
argues against a common carrier (e.g., thiourea) for both the 21 micrometer
feature and the 30 micrometer feature (otherwise these two features should
correlate). This also does not support large PAH clusters as a possible carrier
for the 21 micrometer feature.
