Differences between nominal mix and design mix

Nominal mix	Design mix
The concrete mix in which ratio between different components is predefined.	The concrete mix in which ratio between different components is not predefined. it depends upon water cement ratio
eg.  M-5 to M-25 are considered as nominal mix	eg.   M-25, all the grades are considered as design mix

Different components means-  Cement,sand and gravel 

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M-15

M stands for mix and 15 stands for   characteristic strength at 28 days (15N/mm2)