THE TEE IN TCSH

In 1964, DEC produced the PDP-6; the PDP-10 was a later re-implementation. 
It was re-christened the DECsystem-10 in 1970 or so when they brought out 
the second model, the KI10. 

TENEX was created at Bolt, Beranek & Newman (a Cambridge, Mass. think 
tank) in 1972 as an experiment in demand-paged virtual memory operating 
systems. They built a new pager for the DEC PDP-10 and created the OS to go 
with it. It was extremely successful in academia. 

In 1975, DEC brought out a new model of the PDP-10, the KL10; they intended 
to have only a version of TENEX, which they had licensed from BBN, for the 
new box. They called their version TOPS-20 (their capitalization is 
trademarked). A lot of Tops-10 users ("The OPerating System for PDP-10") 
objected; thus DEC found themselves supporting two incompatible systems on 
the same hardware--but then there were 6 on the PDP-11! 

TENEX, and Tops-20 (I hate the shift key) to version 3, had command 
completion via a user-code-level subroutine library called ULTCMD. With 
version 3, DEC moved all that capability and more into the monitor ("kernel" 
for you Unix types), accessed by the COMND% JSYS ("Jump to SYStem" 
instruction, the supervisor call mechanism [are my IBM roots also 
showing?]). 

The creator of tcsh was impressed by this feature and several others of 
TENEX and Tops-20, and created a version of csh which mimicked them.