  In their seminal paper (Phys. Rev.121, 920 (1961)) Sudarshan, Mathews and Rau
investigated properties of the dynamical $A$ and $B$ maps acting on $n$
dimensional quantum systems. Nature of the dynamical maps in open quantum
system evolutions has attracted great deal of attention in the later years.
However, the novel paper on the $A$ and $B$ dynamical maps has not received its
due attention. In this tutorial article we review the properties of $A$ and $B$
forms associated with the dynamics of finite dimensional quantum systems. In
particular we investigate a canonical structure associated with the $A$ form
and establish its equivalence with the associated $B$ form. We show that the
canonical structure of the $A$ form captures the completely positive (not
completely positive) nature of the dynamics in a succinct manner. This feature
is illustrated through physical examples of qubit channels.
