The first is right. The first element has index 0, second index 1, and so forth, with each n:th element having index n-1, up to the 51:st element.

The second is probably right thinking as well, assuming there is an invisible column heading of "0" for the first column and "1" for the second column. It's a fair enough interpretation to think of the first index as telling which column to look in, and the second index as telling which row (of that column).