Well, conceptually it means you limit how much you can extend the length of the slice inside the array, not that the array has a different length. For example,

Well, conceptually it means you limit how much you can extend the length of the slice inside the array, not that the array has a different length. For example, we have: full := []int{0, 1, 2, 3, 4, 5} half1 := full[:3] // [0, 1, 2] half2 := full[3:] // [3, 4, 5] Now, the capacity of half1 is 6, so it can "grow inside" half2, such that: append(half1, 666) // Now, half2 is [666, 4, 5] This may happen even when you are not using append, for example you could do: half1 = half1[:5] and get into half2 again. The new syntax solves this problem such that, after defining half1 as full[:3:3], if you try half1[:5] you will get an error (the length cannot be larger than the capacity) and if you use append, a new array will be allocated, leaving the contents of half2 intact in any case.