A special part of taxonomy is nomenclature. This consists of rules on what names to use. So a taxonomist first decides what does and does not belong in a group, and then uses nomenclature to decide what name this group should have. If a group is made larger or bigger it may get a different name. On the other hand, the same name may refer to a bigger taxon (according to one particular taxonomist) or a smaller taxon (according to some other taxonomist). This means that scientific names are not guaranteed to be stable. Most names are stable, but for some taxa there is no agreement on its name, because taxonomists do not agree what does belong together and what does not belong together.