Think of the first number as the row and the second as the column. I don't know what you mean about X and Y in relation to the array, typically x is across though (X is a cross, geddit!!!) and y is up or down.
So a foo[1][20] array would be 1 row and 2 columns. May as well just be a 2D array in that case...
You can think of a 3D array as going into or out of the page. You can't visualise a 4D or higher dimensional array.
I'm pretty sure [1][20] is 1 row and 20 columns.