<p>Example:</p>

<pre><code>select ename from emp where hiredate = todate('01/05/81','dd/mm/yy')
</code></pre>

<p>and </p>

<pre><code>select ename from emp where hiredate = todate('01/05/81','dd/mm/rr')
</code></pre>

<p>return different results</p>

http://oracle.ittoolbox.com/groups/technical-functional/oracle-dev-l/difference-between-yyyy-and-rrrr-format-519525
YY allows you to retrieve just two digits of a year, for example, the 99 in
  1999. The other digits (19) are automatically assigned to the current
  century. RR converts two-digit years into four-digit years by rounding.
50-99 are stored as 1950-1999, and dates ending in 00-49 are stored as
  2000-2049. RRRR accepts a four-digit input (although not required), and
  converts two-digit dates as RR does. YYYY accepts 4-digit inputs butdoesn't
  do any date converting
Essentially, your first example will assume that 81 is 2081 whereas the RR one assumes 1981. So the first example should not return any rows as you most likely did not hire any guys after May 1 2081 yet :-)