With C arrays why is a[n]==n[a]

With C arrays why is it the case that  a[5] is equal to 5[a].

The C standard, defines the operator [ ] as follows:
   a[b] == *(a+b)

So, a[5] will evaluate to *(a+5) and 5[a] will evaluate to *(5+a).
Arrays are essentially the pointers a is the memory location and a[5] is the value of element five elements away from a.This is equal to offset "a" from "5" elements at the beginning of the address space (5+a). 

And since the addition is cumulative they both are same.

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