Methylene blue is a very commonly used dye. It stains nuclei blue, and is often used for that purpose. Following "ripening", a process involving atmospheric, or chemical, oxidation and the production of the three strongly metachromatic dyes Azure A, B and C. This solution is called polychrome methylene blue and is very valuable for staining rapid frozen sections. Due to it's strong metachromatic staining properties, it can be used for the demonstration of mucins, cartilage, mast cells, etc.. A comparison of these dyes shows the relationship clearly.