I have what seems to be an easy problem which I am unable to solve....

I have a matrix, lets say

A B C D
E F G H
I J K L

and I want to extrat every 6th element reading from left to right, i.e. F, L
etc.....

Any ideas? I though I could put the matrix into a vertical series, i.e.
A
B
C
D
etc....
and then choose every 6th number but I can't even get that far!!!!

Thanks for any help....