Another important library search procedure is the Hybrid Similarity Search. As its name implies, the mass spectral similarity between a library mass spectrum and query is measured by a hybrid match factor. This match factor also ranges between 0 and 999, however, a value of 999 can be attained even when mass spectra are not identical. Hybrid match factors allow for peaks from the library mass spectrum to be shifted by the molecular mass difference (known or estimated) between the compounds producing the compared spectra, DeltaMass. This means that hybrid match factors approaching and including 999 can occur between mass spectra that share matching peaks as well as non-matching peaks that can be explained through a shift by DeltaMass. Typically, these are spectra of analogs with structural modifications that affect only a single fragmentation pathway (cognates) as well as the same isospectral molecules seen with large simple match factors.