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Concept

SI base units

A circular diagram with concentric rings and four small yellow dots at the cardinal points represents the concept of SI base units, illustrating the foundational units used in the International System of Units.

The seven units from which all other measurements in the International System are derived: the second, metre, kilogram, ampere, kelvin, mole, and candela. After the 2019 redefinition, all seven are anchored to fixed numerical values of fundamental constants rather than physical artefacts or laboratory procedures. The kilogram was the last to make the leap.

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