Monadic Transpose means
mat
1 2 3
4 5 6
⍉ mat
1 4
2 5
3 6
Dyadic Transpose means
2 1 ⍉ mat
1 4
2 5
3 6
1 1 ⍉ mat ⍝ leading diagonal
1 5
mat
1 2 3
4 5 6
⍉ mat
1 4
2 5
3 6
2 1 ⍉ mat
1 4
2 5
3 6
1 1 ⍉ mat ⍝ leading diagonal
1 5