Library /sys$common/syshlp/basichelp.hlb Expressions, Truth values  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  In relational expressions, BASIC generates -1 when the  relationship  is
  true  and  0  when it is false.  In logical expressions, BASIC evaluates
  any nonzero value as true; zero is always evaluated as false.  To  avoid
  unanticipated   results,   use  logical  operators  on  the  results  of
  relational expressions.