Beginning in September 1961, the United States create a semi-formal
advisory project for South Vietnam.  Advisory teams, which went by a
number of different titles had no "typical" amount of personnel.
By September of 1967, however, the program had been formalized
and teams refered to as "Mobile Advisory Teams" or MATs.
These teams were standardized aroud a 5-man element.
*
For simplicity's sake, the term MAT was used for this formation
even before it came into actual official usage.
