What isotope is used as the standard in establishing the atomic mass scale?

The isotope that is used as the standard in establishing the atomic mass scale is the Carbon 12 isotope. This isotope consists 6 protons and 6 neutron in the nucleus. And from researches done, it was found to have exactly a mass of 12 per mol which makes it convenient to be used as a standard reference point for the atomic mass scale. Before this isotope was used, there were two scales that was being used. One based on Oxygen and the other was based on hydrogen. However, the use of these two scale made every research confusing since they exist as a mixture of different of isotopes in nature. So, they have to find a way that would lead them to less errors.