How was the initial standard for the kilogram defined in relation to a common substance?

The mass of one cubic decimeter of pure water at the temperature of its maximum density, specified near $4^{\circ}$ Celsius.

While the meter was defined astronomically, the definition for the unit of mass, the kilogram, was established using a stable, readily available substance: pure water. The precise definition chosen was the mass of exactly one cubic decimeter (a volume measurement) of pure water. To ensure maximum consistency, this measurement had to be taken when the water was at its point of maximum density, which scientists determined occurred at approximately $4^{\circ}$ Celsius. This definition provided a stable, reproducible standard for mass that complemented the new standard for length, forming the critical foundation upon which the initial metric system was built.