Hi,

Probably a very silly question, but how can one talk about "two
diagonals" of a matrix?

I thought the diagonal of a matrix is M(i,i), the one going straight
down the middle.

Any help greatly appreciated.

Bull Dog

In addition to other explanations, there is this:
 <http://mathworld.wolfram.com/Diagonal.html>
The diagonal you refer to is the 'main diagonal'.