Image courtesy of NOAA (.gov)
Luke Howard noticed that clouds often have features of two or more categories, such as cirrus + stratus, cumulus + stratus, etc. Based on these observations, he suggested modifications (or combinations) of the core four clouds between categories. This research served as the starting point for the ten basic types of clouds we observe.
Read more at https://www.noaa.gov/jetstream/clouds/ten-basic-clouds