Military symbols evolved out of the First World War from the British in part for secrecy, and to simplify reference to chemicals by something other than a chemical name. These symbols were sometimes applied as marking on weapons to indicate the agent contents. Military symbols constantly changed and had transitory definitions. For example, mustard gas was assigned the military symbol originally HS for "Hun Stuff". Later in the First World War the S in HS signified mustard gas that had about 25% solvent added to it. This was only in England, as HS was adopted as the military symbol by the United States - signifying crude mustard. In the Second World War the purity of mustard gas was improved through distillation, and this purified chemical warfare agent was designated HD. When it was mixed