problem loading another frame

The sample below works. It consists of 4 separate htm files.

Launch 01.htm creates 2 frames _L, _R and _R has a link which when pressed will load 03.htm into _L and 03.htm will load 04.htm into _R.

When all files are in the same directory it works
When 03.htm is stored in a sub-directory (called sub) below the other files, the load

parent._R.location.href = "04.htm"

doesn't work.

In 02.htm I specify (note in href= the addition of sub\

and when you click on the link load 03.htm into _L it loads 03.htm just fine but the script in the header doen't work.
Can anyone help?
Thanks.

<! 00.htm

<HTML>
<HEAD><TITLE>&nbsp</TITLE></HEAD>
<BODY> Opening screen _L </BODY>
</HTML>

<! 01.htm

<! 02.htm

</BODY>
</HTML>

<! 03.htm

parent._R.location.href = "04.htm";

</SCRIPT></HEAD>
<BODY> Load 04.htm into _R </BODY>
</HTML>

<! 04.htm

<HTML>
<HEAD><TITLE>&nbsp</TITLE></HEAD>
<BODY> This is 04.htm in _R loaded from _L </BODY>
</HTML>