As you well know, bacteria are divided into two groups, historically known as Gram positives and Gram negatives (Archaea are all Gram negative as they lack the telling peptidoglycan). By and large, Gram positives have a cell envelope consisting of a single membrane and a thick layer of peptidoglycan. In contrast, Gram negatives usually sandwich their thin peptidoglycan between two membranes. However, exceptions keep arising, punching holes in the classical definitions. Can we do better? Radhey Gupta of McMaster University thinks so. He proposes that all bacteria be divided into two groups, those with a single membrane cell envelope (single skinned), the "monoderms," and the double skinned "diderms." Will this catch on? Well it might, as it is already gaining considerable traction. What do I think? Speaking for myself, I am ready to adopt the "derm" terminology as it is well grounded in scientific reality and sounds quite accessible to the tongue.
But a problem arises: how can you tell which a newly found bacterium is? Strictly speaking, one would have to determine if the cell envelope is single or double skinned, something that requires looking at a thin section under an electron microscope. This may be handy in some labs but not in others. But there are biochemical stand-ins that approach the answer, so a good approximation is possible with simpler tools. And then again, while you wait for those results, you might as well do a Gram stain!