According to the documentation, Transpose with a second argument {1,1} on a square matrix returns the diagonal of the matrix.
m = ArrayReshape[Range[25], {5, 5}];
Transpose[m, {1, 1}] == Diagonal[m]
(* True *)
Is it an arbitrary convention for the Transpose
function, or does it correspond to some mathematical reality? It does not seem like an obvious consequence of the definition of transposition to me.