The range is actually much more flexible than stated. For example these are all valid ranges:
BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING
BETWEEN 10 PRECEDING AND 5 PRECEDING
BETWEEN 1 FOLLOWING AND UNBOUNDED FOLLOWING
BETWEEN 10 FOLLOWING AND 20 FOLLOWING
The only stipulation is that the beginning of the range has to be smaller than the end of the range.
J. Drew Allen
Business Intelligence Analyst