  Intensities of lines in the near-infrared second overtone band (3--0) of
$^{12}$C$^{16}$O are measured and calculated to an unprecedented degree of
precision and accuracy. Agreement between theory and experiment to better than
1 $\permil$ is demonstrated by results from two laboratories involving two
independent absorption- and dispersion-based cavity-enhanced techniques.
Similarly, independent Fourier transform spectroscopy measurements of stronger
lines in this band yield mutual agreement and consistency with theory at the 1
$\permil$ level. This set of highly accurate intensities can provide an
intrinsic reference for reducing biases in future measurements of spectroscopic
peak areas.
