# Why is the segment of the IR spectrum below 1500 wavenumbers called the fingerprint region?

Why is the segment of the IR spectrum below 1500 wavenumbers (cm$^{-1}$) considered as the fingerprint region? It is said that "although the entire IR spectrum can be used as a fingerprint for the purposes of comparing molecules, the 600 - 1400 wavenumber range is called the fingerprint region. This is normally a complex area showing many bands, frequently overlapping each other". However, I don't understand why the range from 600 - 1400 wavenumbers can be particularly complex and is unique to the compounds, for instance, why is the fingerprint region of propan-1-ol and propan-2-ol so different from each other given roughly the same appearance for rest of the spectrum?

Spectra taken from http://www.chemguide.co.uk/analysis/ir/fingerprint.html