SQL Server 2008:
select 1
where 5 between 1 and 7
1 result
select 1
where 5 between 7 and 1
0 results
Based on these results, and the Postgre Docs I would hypothesize that the ANSI Standard is as follows (although I can't find that doc).
a between x and y
==
a >= x AND a <= y
UPDATE:
The SQL-92 spec says (quote):
"X BETWEEN Y AND Z" is equivalent to "X>=Y AND X<=Z"
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