The C standard defines the Array Subscript Operator [] as follows:

```
a[b] == *(a + b)
```

Therefore a[5] will evaluate to:

```
*(a + 5)
```

and 5[a] will evaluate to:

```
*(5 + a)
```

and from elementary math we know those are equal.

This is the direct artifact of arrays behaving as pointers, "a" is a memory address. "a[5]" is the value that's 5 elements further from "a". The address of this element is "a + 5". This is equal to offset "a" from "5" elements at the beginning of the address space (5 + a).