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I'm trying to pass a nested array to a function, such that the function alters the inner array. I was surprised to find that this minimal reproducible example, despite its use of ByRef, seems to pass a copy of the inner array to the function: #include <Array.au3> ; a boring old array Local $aInnerArray[5] = [1, 2, 3, 4, 5] ; a one-element array containing a reference to the other array Local $aOuterArray[1] = [$aInnerArray] ; intention: take a nested array and alter its inner array ; reality: the inner array seems to be getting copied Func ChangeIt(ByRef $aOuter) Local $aInner = $aOuter[0] $aInner[2] = 0 EndFunc ; Expected: [1, 2, 3, 4, 5] ; Actual: [1, 2, 3, 4, 5] ✔ _ArrayDisplay($aInnerArray, 'Before') ; $aOuterArray passed by-ref, should receive reference to $aInnerArray ; Therefore should change $aInnerArray to [1, 2, 0, 4, 5] ChangeIt($aOuterArray) ; Expected: [1, 2, 0, 4, 5] ; Actual: [1, 2, 3, 4, 5] ✘ _ArrayDisplay($aInnerArray, 'After') I suspect that either: the copy is taking place in the first line of the function (I couldn't find a way to access the inner array without first assigning it to a variable though); or ByRef doesn't propagate into inner levels of the data structure being passed, which seems less likely to me. Could someone please point me in the right direction to get this working as intended? Update: the answer ; WRONG: ; a one-element array containing a reference to the other array Local $aOuterArray[1] = [$aInnerArray] The assumption I made about this code is wrong—it actually copies $aInnerArray into $aOuterArray, so there are now two unrelated $aInnerArray instances. It is not possible to store arrays in other arrays by reference. If it is necessary to refer to a mutable array in multiple places, consider holding it in a global variable. Where a collection of mutable arrays needs to be accessed in multiple places (as in my case), consider storing them in a global array and referring to each sub-array by index (also known as the Registry pattern).