The subdiagonal of a square matrix is the set of elements directly under the elements comprising the diagonal. For example, in the following matrix, the diagonal elements are denoted d_i and the subdiagonals are denoted s_i, [d_1 | a_12 | a_13 | ... | a_(1n) s_1 | d_2 | a_23 | ⋱ | a_(2n) a_31 | s_2 | d_3 | ⋱ | a_(3n) ⋮ | ⋱ | ⋱ | ⋱ | ⋱ a_(n1) | a_(n2) | a_(n3) | ... | d_n].

diagonal | Jordan block | superdiagonal | tridiagonal matrix