The index formula returns a cell at a given position within a range or an array. So the function starts off with, where is your array? And my array might simply be A one to A two. It then asks what row number do you want within that array? So if I just put a number two there, it would give me the value within A two. You can add an optional argument here, which is column number. So as well as going row numbers down, you can also go column numbers to the right. So if my array was from A one to B two, I could then say row number two, column number two. And that would return the value within B two, ie, the second row down, and the second column to the right. The benefit of this, it's very flexible. It can grow as your model grows. As long as you are inserting lines or columns within the range, then your array within the function will automatically update. Downsides, it is difficult to audit.