8
oi.
Replace the quoted filename with your own. Can be anything that once included is correct. File extention doesn't matter.
I use .inc since it suggests Include and make sense to me but it doesn't matter. Also, I tell Apache to not serve .inc files so they can't be viewed, stolen, or abused by any crackheads. But that is another story.
<{include file="your_file_name.html"}>
<{include file="whatever"}>