  Let $\fld$ denote a field and $V$ denote a nonzero finite-dimensional vector
space over $\fld$. We consider an ordered pair of linear transformations $A: V
\to V$ and $A^*: V \to V$ that satisfy (i)--(iii) below. Each of $A, A^*$ is
diagonalizable on $V$. There exists an ordering $\lbrace V_i \rbrace_{i=0}^d$
of the eigenspaces of $A$ such that A^* V_i \subseteq V_0 + V_1 + ... + V_{i+1}
\qquad \qquad (0 \leq i \leq d), where $V_{-1} = 0$, $V_{d+1}= 0$. There exists
an ordering $\lbrace V^*_i \rbrace_{i=0}^{\delta}$ of the eigenspaces of $A^*$
such that A V^*_i \subseteq V^*_0 + V^*_1 + ... +V^*_{i+1} \qquad \qquad (0
\leq i \leq \delta), where $V^*_{-1} = 0$, $V^*_{\delta+1}= 0$. We call such a
pair a {\it Hessenberg pair} on $V$. In this paper we obtain some
characterizations of Hessenberg pairs. We also explain how Hessenberg pairs are
related to tridiagonal pairs.
