In a Simple Similarity Search, the mass spectral similarity between a library mass spectrum and the query is measured by a simple match factor which ranges from 0, when the compared mass spectra have no peaks in common, through to 999, when compared mass spectra are identical. Values approaching, but not identical to, 999 are obtained when the compared mass spectra share several major peaks. Ideally, this means the spectra are replicate measurements of one molecule that can then be uniquely identified. However, some isomers and structural analogs may also result in spectra with large simple match factors. We refer to pairs of unique molecules with numerically indistinguishable mass spectra as isospectral.