A(I, J) * X(J)
**That means I = 1 to N then J=1 to N**

A(I, J) * X(J)

**That instrument I = 1 to N then J=1 to N**

=> A(1,1)*X(1) , A(1,2)*X(2) , A(1,3)*X(3) , A(1,4)*X(1) , ==**> This goes in Row Order i,e we can see
(1,1), (1,2), (1,3), (1,4), foreseeing which is Row order**

A(I, J) * X(J)

**That instrument J= 1 to N then I=1 to N**

=> A(1,1)*X(1) , A(2,1)*X(1) , A(3,1)*X(1) , A(4,1)*X(1) , ===**> This goes in Column Order i,e we can see
(1,1), (2,1), (3,1), (4,1), foreseeing which is Column order**