A decision being made at a conference in France this week carries a lot of weight—and that’s not a metaphor, either. (It also carries a lot of mass.)

The kilogram, which until this point has been based on the weight and mass model of a specific block of platinum and iridium called Le Grand K, will be put up for debate Friday at the General Conference on Weights and Measures in Versailles. The pristine metal prototype of the kilogram—relied upon since 1889, housed under heavy security, and specifically designed to resist corrosion—is widely expected to be replaced with mathematical models for measurement, such as the speed of light.

Of the seven main measures used around the world in the metric system, the kilogram is the last to rely on a physical object; others, such as the meter, have changed to similar light-based calculations. (In the meter’s case, per The Associated Press, it’s the length that light travels in a vacuum in 1/299,792,458th of a second.)

There is, of course, an organizational body behind this measurement and the expected change to the kilogram. The International Bureau of Weights and Measures, known by its French initials as BIPM, houses Le Grand K on its premises and is spearheading the big change. In comments to the AP, BIPM Director Martin Milton told the wire service that the shift to a mathematical model is a sign of progress for the organization, which dates to 1875.

“This, if you like, is a moment of celebration because it’s like the last standard remaining from 1875 that will finally be replaced by new innovation,” Milton explained. “Everything else has been recycled and replaced and improved. This is the last improvement that dates back to the original conception in 1875. So that’s a tribute to what was done in 1875, that it’s lasted this long.”

Another organizational body that’s excited about the big shift is the National Institute of Standards and Technology, a U.S. federal agency, which manages its own version of Le Grand K, having received a duplicate of the original weight after signing the Treaty of the Meter in 1875.

“We’re changing a mass realization system that we’ve had for 129 years,” NIST’s Zeina Kubarych told NPR. “It’s a huge event.”

So why change things now? Part of the reason for the move, NPR notes, is the concern that, even with heavy scrutiny—NIST officials avoid touching the metallic block—it’s succumbing to the challenges of age. There’s a slight weight difference between the American and European variants of Le Grand K, which is a problem because that weight is quite literally the one on which all other kilogram measurements are based.

Well, until later this week, that is.

The U.S. version of Le Grand K, the metal block upon which the kilogram has long been based. (National Institute of Standards and Technology/Wikimedia Commons)