Voltage is measured between two points because it is relative quantity.
Explanation:
•	Since voltage or potential  difference is a work done in moving a charge from one point to another while keeping the electrostatic equilibrium:
Vb-Va = Wab/qo  (q node)
Work done is equal to change in potential energy
Voltage is the change in potential energy per unit charge
Voltage is difference in a potential between two points. Where as one point at higher potential and other at lower potential.
Hence voltage is measured between two points

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