(CNN) A kilo is a kilo is a kilo, right?

Wrong. Monday marks World Metrology Day, and this year's edition sees a big change in the way the kilogram unit is defined.

In November last year, scientists and policy makers from around 60 nations voted unanimously to redefine the kilogram, and Monday is the day their decision takes effect.

The new definition is based on the Planck constant -- a physical constant observed in the natural world -- rather than the precise weight of a piece of metal kept under lock and key.

For more than 100 years Paris has been home to Le Grand K -- or the International Prototype Kilogram, as it is officially known -- a block of metal that previously defined the weight of a kilogram.

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