The acronym ‘ETS’ has two interpretations. The first two letters always denote ‘Emission Trading’. The last one, however, stands for ‘System’ or ‘Scheme’. The acronym is in frequent use. The ETS sector refers to plants covered by the system of carbon dioxide emission trading, and the non-ETS sector refers to all other emission sources. It is estimated that the ETS sector is responsible for 50% of European emission. Other sources—e.g. cars—do not have to acquire permits; they need to comply with other requirements. The ETS started to function in the twenty-first century, but its beginning can be traced to the twentieth century.