Anatomy of a Duality

The nature of M-theory on K3 X I, where I is a line interval, is considered, with a view towards formulating a `matrix theory' representation of that situation. Various limits of this compactification of M-theory yield a number of well known N=1 six dimensional compactifications of the heterotic and type I string theories. Geometrical relations between these limits give rise to string/string dualities between some of these compactifications. At a special point in the moduli space of compactifications, this motivates a partial definition of the matrix theory representation of the M-theory on K3 X I as the large N limit of a certain type IA orientifold model probed by a conglomerate of N D-branes. Such a definition in terms of D-branes and orientifold planes is suggestive, but necessarily incomplete, due to the low amount of superymmetry. It is proposed - following hints from the orientifold model - that the complete matrix theory representation of the K3 X I compactified M-theory is given by the large N limit of compactification - on a suitable `dual' surface - of the `little heterotic string' N = 1 six dimensional quantum theories.