Remember the word "arc" in .  The  is for "row" and
the "c" is for "column".

Although the subscript of  looks exactly like the
number thirteen, 13, it really means row 1 and column 3.  It would
be less confusing if they would put a comma between the 1 and
the 3 and write , but that 
apparently is considered to be too much notation, so no comma is 
placed between the row number and the column number.

To indicate all the elements of a 3x3 matrix in the  notation:



Therefore the element  is the element in row 1 and column
3 in the matrix   



is 4.

Edwin