Sealed under a trio of nested glass bell jars, a gleaming metal cylinder sits in a temperature-controlled vault in the bowels of the International Bureau of Weights and Measures in Sèvres, France. Dubbed Le Grande K, or Big K, this lonely hunk of platinum and iridium has defined mass around the globe for more than a century—from bathroom scales to medical lab balances.

But that is all about to change.

On November 16, 2018, representatives from more than 60 countries voted during the 26th meeting of the General Conference on Weights and Measures in Versailles, France to redefine the kilogram. Today, that change finally takes effect. Rather than basing the unit on this physical object, henceforth, the measure will be