Shadow Weave on 4 shafts
Block A 1,2 1 is blue and 2 is white
Block B 3,4 3 is blue and 4 is white
Block C 2,1 2 is blue and 1, is white
Block D 4,3 4 is blue and 3 is white
So while 1,2 and 3,4 threading are giving you vertical lines on the tabby treadles (2&4 with 1&3), and 2,1 and 4,3 threading gives you horizontal lines. The blue lines reverse when you treadle 1&3 then 2&4.
Then when you treadle or raise shafts (1&4 then shafts 2&3) the line directions change for Block A and Block C. Likewise when treadling 2&3 then 1&4 the blue lines reverse in Block B and D. Those two treadles are swapping coloured threads on shafts 1 and 2.
*This is when a picture is worth a hundred words. So much easier to visualize.