Quantitative structure-activity/property relationship (QSAR/QSPR) methods depend on the transformation of molecules' structures to numbers. Molecular descriptors are applied to conduct the transformation of chemicals to numerical representation. There are a lot of different molecular descriptors -some of them attempt to encode properties of the compound, others try to catch the complexity of the molecule etc. There are also molecular descriptors that rely on fragments of molecules eg. extended connectivity fingerprints (ECFP), atom pairs and topological torsion descriptors. In many cases, these types of descriptors transform molecules into vectors of the same length of either zeros or ones -called bits or vectors of the counts of each molecular fragment.