Organisms in the kingdoms Eubacteria and Archaebacteria were previously grouped into a single kingdom called Monera.

This classification was based on the fact that both groups are prokaryotic, meaning they lack a nucleus and other membrane-bound organelles. However, further research revealed significant differences in their cell structures, biochemistry, and genetics, leading to the separation of Eubacteria (now simply Bacteria) and Archaebacteria (now simply Archaea) into separate kingdoms.