Multilevel Operating Systems
The Basic software for all newer computers
is built on the well-established need for standard 
operating systems. This implies that all applications-no
matter how large, complex or time consuming-must 
operate under (or, more precisely, on top of) the standard
system.  Large applications require supervisory 
monitors which handle problems similar to those of
the operating systems, but at a different level.  
Sometimes, still a third or even a fourth such level
is required or desirable.  This leads naturally 
to the concept of multilevel systems-similar vertically,
but different horizontally.  Proper division 
of responsibility between levels leads to greater efficiency and
less logical complexity, while actually 
enhancing capability.
CACM March, 1966
Orchard-Hays, W.
